Guidance on Signal Positioning and Visibility

Transcription

1 Guidance on Signal Positioning and Visibility Synopsis This document provides guidance on the requirements for positioning signals and indicators to ensure adequate viewing and clarity of meaning for drivers. Signatures removed from electronic version Submitted by Paul Woolford Standards Project Manager Authorised by Anne Blakeney Acting Department Head Railway Group Standards Management This document is the property of the Rail Safety and Standards Board Limited. It shall not be reproduced in whole or in part without the written permission of the Department Head of Railway Group Standards Management, Rail Safety and Standards Board. Published by: Rail Safety and Standards Board Evergreen House 160 Euston Road London NW1 2DX Copyright 2003 Rail Safety and Standards Board Limited

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3 Page 1 of 71 Contents Section Description Page Part A A1 Issue record 2 A2 Implementation of this document 2 A3 Responsibilities 2 A4 Health and safety responsibilities 2 A5 Technical content 2 A6 Supply 2 Part B B1 Purpose 3 B2 Application of this document 3 B3 Definitions 3 B4 Introduction 6 B5 Requirements 6 B6 Detailed requirements 12 B7 Signal sighting process 13 B8 Commissioning checks 22 B9 Records 23 Part C C1 General requirements for positioning and visibility of signals 25 C2 Colourlight main signals and associated route indicators 34 C3 Position light signals (PLS) and associated route indicators 38 C4 Semaphore signals and signs that performs the function of signals 40 C5 Use of co-acting signals and banner repeating signals 42 C6 Platform equipment 45 C7 Miscellaneous technical requirements 49 Appendices 1 Required reading distance summary of issues 54 2 Typical forms for recording signal sighting details 57 3 Human vision and perception 58 4 Cab vision plots 70 References 71 RAIL SAFETY AND STANDARDS BOARD 1

4 Page 2 of 71 A1 Issue record Part A Issue Date Comments One December 2003 Original document This document will be updated when necessary by distribution of a complete replacement. A2 Implementation of this document The publication date of this document is 6 December This document does not supersede any other s. A3 Responsibilities s are non-mandatory documents providing helpful information relating to the control of hazards and often set out a suggested approach, which may be appropriate for Railway Group* members to follow. * The Railway Group comprises Network Rail Infrastructure Limited, Rail Safety and Standards Board Limited, and the train and station operators who hold railway safety cases for operation on or related to infrastructure controlled by Network Rail Infrastructure Limited. Network Rail Infrastructure Limited is also known as Network Rail. Rail Safety and Standards Board Limited is also known as RSSB. A4 Health and safety responsibilities A5 Technical content In issuing this document, RSSB makes no warranties, express or implied, that compliance with all or any document published by RSSB is sufficient on its own to ensure safe systems of work or operation. Each user is reminded of its own responsibilities to ensure health and safety at work and its individual duties under health and safety legislation. The technical content of this document has been approved by: Jeff Allan, Principal Signalling and Telecommunications Engineer, RSSB Richard Evans, Principal, Operations, RSSB Enquires to be directed to RSSB Tel: or enquiries@rssb.co.uk. A6 Supply Controlled and uncontrolled copies of this document may be obtained from the Industry Safety Liaison Dept, Rail Safety and Standards Board, Evergreen House, 160 Euston Road, London NW1 2DX or enquiries@rssb.co.uk. 2 RAIL SAFETY AND STANDARDS BOARD

5 Page 3 of 71 B1 Purpose Part B This document supports and provides guidance on the requirements for positioning signals and indicators to ensure adequate viewing and clarity of meaning for drivers. The complete text from is shown boxed and further explanation of the requirements is given in this Guidance Note. B2 Application of this document B2.1 To whom the guidance applies This document contains guidance that is applicable to RSSB and duty holders of the following categories of Railway Safety Case: a) infrastructure controller b) station operator c) train operator. Most of the requirements of should, in practice, be carried out by the signal sighting committee. Within the structure of Railway Group Standards, however, all requirements have to be placed on one or more Railway Group members. Potentially this could result in unhelpfully complex sentences introducing each requirement. Clauses B2.2.1, B2.2.2 and B2.2.3 of have been written as overarching requirements for the whole standard, as a means of keeping the main point of the document as clear as possible. B2.2 Documents supported by this Guidance Note Signal Positioning and Visibility. B3 Definitions and acronyms The following definitions and acronyms are common to and this Guidance Note. B3 Definitions and acronyms Achievable reading distance The maximum reading distance that can be reasonably practicably achieved. Alignment The horizontal and vertical relationship between the centre-line of an element of a signal and the railway infrastructure. Assessed minimum reading time The assessed minimum reading time is the sum of the times assessed to be essential in order for a driver approaching a signal to: a) detect the presence of the signal b) identify the signal as being applicable to the driver c) observe the information presented by the signal d) interpret the information to determine what action, if any, is required. RAIL SAFETY AND STANDARDS BOARD 3

6 Page 4 of 71 AWS Automatic Warning System. CD Close door. Centre-line The line (usually extending out perpendicularly from the face of a signal) forming the optical axis of the beam. DOO Driver only operation. Driver s eyelevel The height above rail level that is assumed for the driver s eye. This will vary according to the design of the cab, the seat adjustment and the driver. For the purposes of signal sighting, the driver s eyelevel shall normally be assumed to be 2.75 m above rail level unless sighting exclusively for trains of a non-standard height. Driving position The normal position from which the driver controls the train, by operating the primary controls (see GM/RT2161 for further details). Element An element of a signal is any of the following: a) a single aspect of a main signal (a double yellow counts as one element) b) a position light junction indicator c) the number/character displayed by an alphanumeric route indicator d) a single arm/disc of a semaphore signal or, during darkness, the illuminated spectacle e) an aspect of a position light signal (the two white lights counts as one element). Form of signal The combination of elements used to create a signal, and their positions relative to each other. Lateral position The horizontal distance from the running edge of the nearest rail of the line to which the signal applies at a specific location on the track, and the vertical position above rail head level. Longitudinal position The position along the track. Normal stopping position Any position within the range of positions at which drivers are expected to stop on the approach to a signal at danger. OLE Overhead line equipment. Permissible speed The highest permitted speed (including any enhanced permissible speed) of a train on the approach to a signal. 4 RAIL SAFETY AND STANDARDS BOARD

7 Page 5 of 71 Platform starting signal A signal is defined as a platform starting signal if either: a) the longest passenger (or empty coaching stock) train authorised to use the platform would, if stopped at the signal, still have a portion of the passenger accommodation alongside the platform, or b) the leading end of the train passes over the automatic warning system (AWS) equipment associated with the signal before it is dispatched from the station. PLS Position light signal. PLJI Position light junction indicator. RA Right away. Readable An aspect or indication is readable if, under clear weather conditions by day and by night, persons meeting the minimum eyesight requirements of Railway Group Standards GO/RT3251 or GO/RT3255 are always able to identify the aspects and indications displayed when viewed from the driving position. Clear weather conditions means daylight visibility of 1000 m or better, where visibility is measured in accordance with guidelines such as those set out in the British Meteorological Office Observer s Handbook. GK/RT0031 sets out the minimum requirements for signals. Required reading distance The distance before the signal determined by the signal sighting committee that is to be provided and maintained (see GE/RT8034) for sighting a signal by the driver of an approaching train. SAS SPAD Start against signal SPAD. Signal A visual display device which conveys instructions or provides prior warning of instructions regarding the driver s authority to proceed. For the purposes of this standard it includes level crossing indications to train drivers, points and SPAD indicators, and signs or boards that perform the function of a fixed signal. Signal file The records associated with all signal sighting issues for each signal. SOY SPAD Start on yellow SPAD. SPAD Signal passed at danger. SSC Signal sighting committee. Other definitions and acronyms are as defined in GK/RT0002 (due to be re-issued as GK/GN0802 on 3 April 2004) No guidance is provided. RAIL SAFETY AND STANDARDS BOARD 5

8 Page 6 of 71 B4 Introduction B4 Introduction () On a railway where driver observation of lineside signals and signs is essential to the correct control of trains, driver cognition and response factors have the potential to introduce some significant risks. Even in the case of a perfectly positioned signal there is a finite risk of a Signal Passed at Danger (SPAD). In situations where there are particular risk factors associated with the signal (such as conspicuity problems, restricted approach view, interruptions to the view), then each of these factors constitutes a departure from the ideal and thereby increases the SPAD risk. Where it is necessary to position signals in a manner which departs from the ideal, the signal sighting committee shall assess the risk implications of each departure from the ideal and shall recommend the form and position of the signal that: a) most nearly meets the requirements of this Standard b) reduces the risk of SPAD to as low as reasonably practicable c) ensure that no intolerable risks are introduced. This document supports HM Railway Inspectorate Safety Principle 20: The signalling system should provide for the safe routing, spacing and control of trains. The process of selecting the position, form and alignment of signals is a complex process which requires considerable judgement. An ideal arrangement, one that meets all the requirements of, may not be achievable. It is the duty of the signal sighting committee to consider how to implement the signalling design in such a manner that best fits the requirements of and does not introduce intolerable risk. In particular, the signal sighting committee should identify the best balance between different requirements that can appear conflicting. Additionally, the signal sighting committee should understand that multiple departures from the requirements of Part C may well introduce significant additional risk, either when the multiple departures relate to one signal or to successive signals between the first caution and the associated red aspect. Part C of represents best practice for positioning signals on an ideal railway. Unfortunately the real world introduces complications that may make applying the standard unachievable. The signal sighting committee should therefore make judgements about how and when departures from the requirements of Part C are appropriate and acceptable, bearing in mind that each departure is likely to introduce some additional risk. This Guidance Note (along with other material) is intended to help understand the potential risks associated with departures from each clause of Part C, so that a reasoned argument can be put for any particular solution proposed, as required in clause B6 of the standard. B5 Requirements B5.1 Principal requirements Signals shall be positioned and aligned so as to ensure that: a) the driver of an approaching train has sufficient time to identify, observe and interpret the information being displayed 6 RAIL SAFETY AND STANDARDS BOARD

9 Page 7 of 71 b) the information being presented is clear and unambiguous c) the risk of reading the wrong signal is minimised d) the presentation of information displayed to the driver is such as to avoid information overload. The principal requirements identified are the high-level objectives that should be achieved for every signal. Whilst the requirements in Part C of have been developed to achieve these objectives for most circumstances encountered on the railway, it is acknowledged that there will always be exceptions to the rule. The structure of now gives more scope to deviate from the norm where this is felt appropriate; but the four listed principal requirements represent the fundamental requirements and any departure from the normal arrangement should be tested against these to ensure that the proposed alternative is acceptable for the specific circumstances identified. The following points should assist when determining the acceptability of proposed alternative arrangements. a) Sufficient reading time: This is an immensely complex issue. It should be recognised that there are many factors relating to the signal s position and appearance that could affect the amount of time needed; that the driver s response may be affected by circumstances beyond the control of the signal sighting committee; and that different drivers behave in differing ways. Inevitably there is no right answer, but the test for sufficiency should be based on what other departures exist that might be increasing the risk of error and the consequences of any error. b) Clear and unambiguous information: Ensuring that the information presented can only have one meaning (unambiguous) is normally achieved through consistency of appearance and position. Expectation of an approach-controlled signal clearing every time can, in the mind of the driver, generate an unintentional meaning resulting in ambiguous information. c) Minimising the risk of mis-read: Mis-reading is clearly an issue where parallel signals exist but can also be an issue with non-parallel signals on other lines or reading through to another signal on the same line. d) Avoiding information overload: Although some signals are quite complex in form, the issue of information overload is most likely to result from observing signals in an environment full of other signage, particularly permanent and temporary speed signage. In most cases, moving or removing the signage should be preferable to moving the signal. Audible alarms may also contribute to information overload. B5.2 Assessed minimum reading time B5.2.1 Determining when an assessed minimum reading time is required Except as permitted below, the assessed minimum reading time, as defined in section B3, shall be determined for each signal. RAIL SAFETY AND STANDARDS BOARD 7

10 Page 8 of 71 An assessed minimum reading time is not required for the following types of signal: a) signals where trains can only start from rest (for example a terminal platform starter) b) signals that can only display a stop aspect (for example a fixed red or a stop board) c) signals where all approaching trains are required to proceed at a speed which allows them to stop on sight (for example, independent PLS). For signals not requiring an assessed minimum reading time, a minimum reading distance shall be determined that ensures that any approaching train is capable of stopping at the signal. There are categories of signal for which the need to determine a reading time is inappropriate. Three specific categories are identified which require different approaches in order to determine an acceptable reading distance: a) A train in a terminal or bay platform should not set off until the driver has observed a proceed aspect. Therefore, assuming that the driver of any train can observe the signal (or an indication of the signal) before setting off, as set out in clause C6.1.1, no other reading distance should be considered. b) When approaching a signal displaying a stop aspect, the driver requires to observe the signal from a distance sufficient to judge the braking so as to stop the train just short of the signal, typically 180 m. If the signal can only display a stop aspect, then no minimum reading time assessment is needed. c) Some signals (principally those used for shunting) require the train speed to be controlled so that the train can stop short of any obstruction (referred to as proceed on sight ). Signals approached in this manner require to be read from a distance that allows the train to be stopped short of the signal, typically 50 m. B5.2.2 Determining the assessed minimum reading time The assessed minimum reading time shall be no less than eight seconds travelling time before the signal. The assessed minimum reading time shall be greater than eight seconds where there is an increased likelihood of misread or failure to observe. Circumstances where this applies include, but are not necessarily limited to, the following: a) the time taken to identify the signal is longer (for example, because the signal being viewed is one of a number of signals on a gantry, or because the signal is viewed against a complex background) b) the time taken to interpret the information presented by the signal is longer (for example, because the signal is capable of presenting route information for a complex layout ahead) c) there is a risk that the need to perform other duties could cause distraction from viewing the signal correctly (for example, the observance of lineside signs, a station stop between the caution and stop signals, or DOO (P) duties) d) the control of the train speed is influenced by other factors (for example, anticipation of the signal aspect changing). 8 RAIL SAFETY AND STANDARDS BOARD

11 Page 9 of 71 The assessed minimum reading time shall be determined using a structured format approved by the infrastructure controller. Clause B5.2.2 requires that an individual assessment be carried out for each signal to determine if there are any site specific hazards that may result in the signal reading taking longer. The use of a minimum reading time assessment form has become common even though the actual format was not specified. now calls for the infrastructure controller to approve a structured format for this use. In this way, the format should be consistent across the network whilst allowing improvement to take place without the need for a revision to the Railway Group Standard. An understanding of human factors issues regarding the variation in time needed for a driver to identify, observe and interpret the signal information is now an important part of assessing the suitability of a proposed signal position, form and alignment. Several factors that affect this have now been identified and the minimum reading time (MRT) assessment carried out depends on an understanding of these. The process chart in Appendix 1 identifies a typical sequence of events that may be needed before the signal sighting committee agree to the reading time recorded for a particular signal. The human factors issues in Appendix 3 identify features that may require additional reading time. B5.3 Interruption of view So far as is reasonably practicable, there shall be no interruption of the driver s view of a signal during the minimum reading time. An assessment shall be made of any remaining interruptions to demonstrate that they do not give rise to an unacceptable risk of the driver disregarding or misreading the signal. When determining where the minimum reading time commences, referred to as the minimum reading distance (MRD), a distance equivalent to the length of any interruptions shall be added. To meet the requirements of clause B5.3 the infrastructure controller should develop and publish a set of criteria that can be used to determine the acceptability of any interruptions that cannot be removed. It is recommended that this assessment includes a means of recording interruptions as a plot of sections of the approach which are affected. This should then be subjected to the criteria to determine acceptability and to identify any further improvements necessary. When assessing the acceptability of interruptions, particular attention should be given to sequences of regular interruptions (typically OLE stanchions on a lefthand curve) that could create the effect of a flashing aspect. It is also recommended that the final approach (typically 180 m) to a signal capable of displaying a stop aspect be free of interruptions to aid the final braking to a stand. The minimum reading time assessment assumes that the signal is readable for the calculated time period. If part of that is lost through obstructions, this lost time should be added back into the requirement, with the result that the start of the minimum reading time is further away from the signal. B5.4 Calculating the location at which the assessed minimum reading time commences The location on the train s approach to the signal at which the assessed minimum reading time commences (known as the minimum reading distance) shall usually be calculated using the permissible speed applicable at that location. RAIL SAFETY AND STANDARDS BOARD 9

12 Page 10 of 71 Where the maximum attainable speed of all trains on the approach to the signal is less than the permissible speed, it is permissible to calculate the minimum reading distance based on attainable speed. It is permissible to determine separately the assessed minimum reading time (and hence minimum reading distance) for each aspect that the signal can display. With the signal displaying a clear or first caution aspect the permissible speed (or attainable speed if justified) shall be used to determine the minimum reading distance. For other aspects it is permissible to determine the minimum reading distance based on the attainable speed of trains as appropriate to the aspect being displayed (see also clause B5.2.1). It is permissible to determine the minimum reading distance of semaphore stop signals based only on the approach speed to the red aspect. Whilst use of attainable speed as opposed to permissible speed is an accepted option, as described in the second paragraph of clause B5.4, there are risks involved. Firstly, the very fact that attainable speed is being considered suggests that the assessed reading time (at permissible speed) cannot be achieved and therefore reading time is not likely to be ideal. Secondly, it is likely that the signal is in a location where the driver is required to accelerate or decelerate. Both have the potential for being a distraction from obeying the signal. Thirdly, there should be a mechanism in place to ensure that, as new trains are introduced with better acceleration or braking, the assumptions made to justify the use of attainable speed are not infringed. This may be difficult to achieve over the lifetime of a signal. Assessing the minimum reading distance for each aspect displayed recognises that not all elements of the signal need the same reading distance requirements. For example, where a junction signal is approach-controlled from red the minimum reading distance of the route indication (but not of the main signal itself) can be based on the speed profile of the train as it approaches the signal at danger. (With semaphore junction signals, all arms/lights should be read whichever route is set for the driver to determine the route that has been set.) Similarly a subsidiary aspect should only be observed by trains approaching the signal at a speed appropriate to being able to stop. Assessing each aspect separately may also be helpful if the achievable reading distance is initially determined as less than the minimum reading distance (see clause C1.3.1). Additional time added to mitigate against identified hazards (see clause B5.2.2) should only be applied to those assessments where the hazard is considered appropriate. For example, hazards associated with station duties need not be applied when the train is travelling at the permissible speed. Semaphore stop signals historically have been positioned with short reading distances. This may still be acceptable because a driver should have received sufficient information at the distant signal to know either that the stop signal is off or that the train speed should be reduced before approaching the stop signal. Hence the reading distance may be calculated using a much reduced speed. B5.5 Maximising the reading distance achievable As far as is reasonably practicable, hazards (identified in the requirements set out in clause B5.2.2) and obstructions to the driver s view of the signal shall be removed so as to maximise the reading distance achievable (known as the achievable reading distance) and minimise hazards that could increase the risk of driver error. 10 RAIL SAFETY AND STANDARDS BOARD

13 Page 11 of 71 Where alterations have been made that affect hazards identified, the assessed minimum reading time shall be reviewed. The reading distance given to the driver should be as large as is reasonably practicable. Judging what is reasonably practicable is not always easy; removing lineside vegetation is relatively easy to achieve, whilst trees outside the railway boundary may not be. What is reasonably practicable for a new signal may not be for an existing signal (for example, for a new signal it is possible to reconsider its location to improve the reading distance achievable). Most importantly, the effort made to increase the reading distance should be far greater if this is less than the assessed minimum reading distance, than if the achievable reading distance is already greater than the minimum. B5.6 Form of signals The form of approved signals are set out in GK/RT0031. Although GK/RT0031 is in place to regulate the style and appearance of all approved signal types, there could be occasions when new or experimental forms of signals are applied which are not yet covered by the standard. Signal sighting committees may wish to take advantage of these new options to help solve specific problems identified, but the infrastructure controller should carefully control the use of these new devices until they are formally approved. A derogation or non-compliance, pending standards change, is likely to be required against GK/RT0031. At the time of publication preliminary route indicators (PRI) and miniature main aspect units are being appraised. B5.7 Longitudinal positioning of signals The longitudinal position of signals shall be selected so as to ensure that they meet the readability requirements set out elsewhere in this standard, whilst also taking into account the requirements for: a) signal spacing (see GK/RT0034) b) the provision of signals (see GK/RT0032) c) the positioning of signals used to control movements onto occupied lines (see GK/RT0044). In order to meet the readability requirements of it may be necessary to reposition signals longitudinally from their designed position. Whilst this may be essential to achieve an acceptable reading time, it may introduce uneven spacing between signals. The driver s role of judging where to stop a train is partly based on the spacing of signals recently passed by the driver. If the signal spacing is consistent, then a driver can have confidence in where the stop signal should be even before being able to sight it. If the signal spacing is inconsistent, then a driver may be mistaken as to where the stop signal is and consequently misjudge the train braking. GK/RT0034 provides tolerances on variations in signal spacing, but it is likely that irregular spacing becomes more critical when the driver is out of sight of any signal for a large part of the travelling time. Other factors that can interfere with the need for regular spacing include changes in gradient and changes in permissible speed. RAIL SAFETY AND STANDARDS BOARD 11

14 Page 12 of 71 B5.8 Structural and electrical clearances Requirements for structure clearances relating to the positioning of new, altered and temporary infrastructure are set out in GC/RT5212. Additional requirements for personal safety in respect of clearances are set out in GC/RT5203. Signals and signal structures shall be positioned so as to meet the requirements of GE/RT8025. No guidance is provided. B5.9 Restrictions on use of co-acting signals The co-acting signal shall be of the same style (colourlight or semaphore) as the primary signal. Section C5.2 identifies further factors to address when considering the use of a co-acting signal. No guidance is provided. B5.10 Maintainability The design of the signal and associated structure shall take into consideration the proposed arrangements for the maintenance and alignment checks of the signal, and shall incorporate any features necessary to allow access to the equipment for such purposes. Legislation and standards on safe access can create significant restrictions on the positioning of signals. In general, the result is to place supporting structures further to the side or higher above the rails than might otherwise be desirable. It is important to make sure that the positioning of signals is not compromised by difficulties of providing access. Equally, it is important that facilities for access are included. New signal head technologies can help by reducing the maintenance requirements or by separating the parts that need regular access from the aspect display units. Similarly, it may be possible to utilise structures that bring the signal head into a maintainable position. The access route need not be a permanent structure, particularly if the frequency of access or urgency of obtaining access is sufficiently low. If access can be restricted to times when the line can be blocked, then temporary facilities such as access platforms may be adequate. B6 Detailed requirements B6 Managing departures from best practice The contents of Part C of this standard represent current best practice for the appearance, sighting and alignment of signals. Signals shall usually be positioned in accordance with Part C; however, it is permissible not to comply with all the details stated, but any departures from the requirements of Part C shall be permitted only if: a) supported by all disciplines represented on the signal sighting committee b) approved by the infrastructure controller c) the details and justification of the departures from the requirements of Part C are recorded in the signal file. 12 RAIL SAFETY AND STANDARDS BOARD

15 Page 13 of 71 The infrastructure controller shall review all such departures to ensure that the rationale being applied is consistent, logical, justified and complies with the principal requirements identified in clause B5.1. The application of this clause B6 does not absolve Railway Group members from seeking derogations, temporary non-compliances or non-compliances pending standards change from other relevant Railway Group Standards. provides for situations where the detailed requirements listed in Part C of the standard are not appropriate to the particular circumstances faced by a signal sighting committee. It is possible for the signal sighting committee to use its judgement and experience to depart from these detailed requirements, so long as the process described in section B6 is complied with. A signal sighting committee can have a difficult job determining how much effort and cost is justified in trying to meet all the requirements of the signal sighting standard. Whilst introducing a mechanism to permit departures from the detailed requirements, recognises the need to keep close control over how these departures are permitted. Furthermore, the signal sighting committee should understand that in general the number of departures needed should be small, and circumstances where multiple departures are being considered should be assessed extremely carefully before recommending such a course of action. B7 Signal sighting process The first requirement is that all disciplines of the signal sighting committee support the proposed departure. This is intended primarily to ensure that the driver s representatives have to be supportive of a departure that might result in a non-standard presentation of information to their drivers. The second requirement is that the infrastructure controller, as owner of the signalling, approves the proposed departure. In particular, the infrastructure controller should be able to demonstrate that the proposal still meets the principal requirements set out in and that there is a process in place to ensure that, so far as is appropriate, departures are applied consistently across the network. The third requirement is that the details and justification of the departure are recorded on the signal file. The main purpose of which is to ensure that the reason for the departure is logged for future reference. There are many existing signals which have been installed with unusual features, such as being positioned on the right-hand side or aligned in a non-standard way. Without documented reasons, the purpose of these anomalies are unclear, making future decisions regarding the suitability of the signal s sighting less well founded. For the infrastructure controller to manage the process of approving and justifying departures, there should be some review processes in place which can establish national consensus on acceptable departures. It may be possible for some more common departures to be covered by company standards as a means of ensuring consistency and identifying what is considered acceptable. B7.1 Concept design stage requirement Consideration of signal sighting issues shall be properly co-ordinated with the overall scheme design process and shall take place sufficiently early in a scheme that the options available to a committee to optimise the readability of signals are not restricted by actions or decisions already made. As a part of the signalling design production and overrun risk assessment processes (see GK/RT0207 and GI/RT7006 respectively), as much detail as possible about the proposed position and form of each new and altered signal (and each indicator and sign performing the function of a signal) shall be produced during the concept design phase. RAIL SAFETY AND STANDARDS BOARD 13

16 Page 14 of 71 These details shall be submitted to a signal sighting committee in order for an onsite evaluation of the proposals to be undertaken. It is also permissible to use equipment such as virtual reality simulation or video facilities to support the signal sighting design process. Completion of an approved signal sighting form should be thought of as a design process with multiple steps and iterations in which the form represents the final approved design (see Figure 1). Track Layout Mandatory requirements (Signals, overlaps, train protection etc.) Operational requirements Conceptual signalling design (GK/RT0207) Layout risk assessment (GI/RT7006) Amend track, signalling and operational use as necessary [*] * Arrowed line depicting "recycling" of signalling, track and operational requirements following risk assessment and signal sighting are not shown. Signal sighting () Detailed signalling design (GK/RT0207) Figure 1 Signalling design process At the concept design stage the signal s appearance and longitudinal position is determined purely on the basis of the track layout, braking performance, interlocking rules and operational requirements. It is not normally practical to carry out a full signal sighting assessment at this stage, since other factors such as the layout risk assessment may alter the requirements for the signal position and appearance. However, it should be noted that the detailed design does need (at least in part) the signal sighting details to be considered, and it is recommended that a preliminary assessment is undertaken to determine the practicality of the proposed signalling arrangement. does not mandate the involvement of the signal sighting committee at this stage; however, the inclusion of train operator representatives at this stage has proved to identify problems and solutions earlier in the project life. Proposals made by the signal sighting committee can in some instances require the design of signalling plans, circuitry and, possibly, operational flexibility or electrification equipment to be altered. Clearly, any such changes to the scheme design could have financial and timescale implications unless identified at an early stage of the scheme. Where possible, therefore, issues such as achieving an acceptable reading time or the need to provide additional equipment and structures, should be identified as early as possible in the scheme design process. 14 RAIL SAFETY AND STANDARDS BOARD

17 Page 15 of 71 The degree of impact on signal sighting issues likely to be encountered at the concept design stage and during layout risk assessment varies widely. For plain line sections of track there is not likely to be much impact at all; however, the more complex the layout the greater the interaction between issues of scheme design and signal sighting. There are now several video/software packages that can be used to visualise the proposed signal position and form, so that early assessments of reading time and clarity can be carried out. It is not yet considered that such visualisation packages can replace the need for a site assessment, but they do provide a powerful tool to enable options to be considered at an early stage in the scheme design and can also reduce the amount of time needed to be spent out on the track. If the layout is to be significantly modified or is entirely new, such visualisation techniques become essential as a means of predicting what the appearance of the signals is going to be on a track layout that does not yet exist. Similarly, the impact of new OLE or structures on signal sighting can be successfully assessed. B7.2 Role of signal sighting committees B7.2.1 New, modified or repositioned signals For all new, modified and re-positioned signals, indicators, and signs that perform the function of signals, the infrastructure controller shall ensure that a signal sighting committee is convened to verify the proposed position and form. The committee shall: a) review any preliminary sighting proposals produced by the designers and the signalling scheme plan b) identify any alterations or additions required c) make recommendations as to the position and form of each signal in order to optimise the driver s approach view. The infrastructure controller shall ensure that where multiple signals are involved, a consistent approach to signal sighting is taken for all signals. Wherever practicable this shall be achieved by using the same personnel for sighting all the signals. When satisfied, each committee member shall indicate in writing their agreement with the proposed sighting details, and the committee shall recommend the sighting details for approval by the infrastructure controller (see clause B7.6). B7.2.2 SPAD investigations For existing signals subject to a SPAD investigation, the infrastructure controller shall ensure that the investigation remit set by the Inquiry Panel Chairperson (see GO/RT3252) is performed by personnel competent in signal sighting. It is permissible for the committee membership to be restricted below that set out in clause B7.4.1 so long as the committee is capable of fulfilling the investigation remit set. B7.2.3 Linespeed increase For existing signals subject to a proposed linespeed increase, the infrastructure controller shall ensure that a signal sighting committee is convened to: a) assess the signal for compliance with this standard at the proposed speed b) where necessary, make recommendations regarding improvements that shall be made to the signal before introduction of the higher speed. RAIL SAFETY AND STANDARDS BOARD 15

18 Page 16 of 71 When satisfied, each committee member shall indicate in writing: c) their agreement that no change is required, or d) their agreement with the proposed changes. See also GK/RT0007 and GI/RT7006. The standard suggests that there are three distinct roles for a signal sighting committee, depending on whether the work is related to new or modified signals (clause B7.2.1), SPAD investigations (clause B7.2.2) or linespeed increase (clause B7.2.3). Fundamentally, the role is, in fact, the same in all three cases, that of assessing the signal or proposed signal and demonstrating that it is, or will be, fit for purpose. This can be achieved by demonstrating that the signal is, or will be, compliant with clause B5.1. Where the signal is new, there is scope to challenge the position and form proposed by the signal design, and this greater freedom makes signal sighting of new signals technically more complex. Where signals already exist, there is greater pressure not to change the signal position or appearance. It is also likely that the signal was originally installed when previous signal sighting standards were in force, such that the assessment identifies features that are not in line with Part C of. In these circumstances the signal sighting committee should assess the signal to determine what departures exist, and whether these departures should be considered acceptable. If these departures are not acceptable, then the committee should consider how the signal can be improved and make recommendations that should achieve the needed improvements. If the signal is considered acceptable, the signal sighting committee may recommend that no improvements are necessary. However, if they judge that the signal is only just acceptable, and reasonably practicable measures can be identified to improve the signal, these should be recommended. In the case of a SPAD investigation the detailed remit should be set by the investigation panel based on the facts available. Not all cases require a comprehensive signal sighting committee to attend and the standard allows for resources appropriate to the circumstances to be deployed. GO/RT3252 and its supporting Guidance Note, GO/GN3652, provide considerable information regarding issues to be considered following a SPAD. In particular, it is recognised that attendance soon after the SPAD has occurred to capture perishable evidence, when combined with information from the signal file, may avoid the need for a full signal sighting committee to be convened. In the case of a linespeed increase, it should be noted that GI/RT7006 also requires assessments of the risks involved. The recommendations of the signal sighting committee should be judged in the context of the overrun assessment, particularly with regard to determining what departures are acceptable. B7.3 Additional responsibilities of the signal sighting committee A signal sighting committee shall also be required to consider the positioning of the following items of infrastructure equipment: a) signal post telephones (see GE/RT8048) b) permanent speed boards (see GK/RT0038) c) AWS inductors (see GE/RT8035) 16 RAIL SAFETY AND STANDARDS BOARD

19 Page 17 of 71 d) platform/track mounted monitors and mirrors to be used by drivers (see GE/RT8060). Features other than signals have a role in reducing the risk of driver error when controlling the train s speed. Some features such as signal post telephones and platform monitors are relevant to the stopping position of trains and hence the prevention of SAS SPADs. Other features such as speed boards and AWS inductors are relevant to trains in motion and should be controlled so that the sequence and quality of information presented to the driver is acceptable. Railway Group members, particularly the infrastructure controller, may occasionally identify other items of equipment whose positioning could benefit from the expertise available within the membership of a signal sighting committee. does not mandate their involvement, but where input from multiple Railway Group members is appropriate, a committee similar to a signal sighting committee may be helpful. B7.4 Signal sighting committee core competence requirements B7.4.1 Committee membership The signal sighting committee shall collectively include all the expertise (engineering, operations, train driving, local knowledge) necessary to assess the signals in question. In particular, the infrastructure controller shall ensure that the committee includes: a) representatives of the infrastructure controller, with both operations and signal engineering experience. Other engineering expertise shall be included where necessary b) a representative with knowledge of the signalling scheme design, unless the committee has been convened to assess a SPAD incident c) representatives of all train operators using the route, with experience in driving the relevant types of trains d) one or more representatives of station operators on the route, if the signals under consideration include platform starting signals. The infrastructure controller shall consider including other specialist skills within the committee, such as human factors, where the membership would otherwise not contain sufficient skills for the specific task in hand. The infrastructure controller shall consider including members of an investigation/inquiry panel within the committee where the committee is formed as a result of an accident or incident. B7.4.2 Competence standards Those members who bring expertise of signal engineering, drivers skills or operations requirements to the committee, together with the chairperson, shall be assessed as competent in the core requirements of a signal sighting committee. The signal sighting committee chairperson shall, additionally, be assessed as competent in the core requirements of a signal sighting committee chairperson. It is permissible for a SSC chairperson to be the representative of an area of expertise in addition to the chairperson s duties. The infrastructure controller shall set and monitor competence standards for the core requirements of a signal sighting committee and for the core requirements of a signal sighting committee chairperson. The infrastructure controller shall maintain a central register of personnel assessed as competent in the core requirements of a signal sighting committee or the core requirements of a signal sighting committee chairperson. RAIL SAFETY AND STANDARDS BOARD 17

20 Page 18 of 71 The committee shall be led by a chairperson, appointed and approved in advance by the infrastructure controller. The infrastructure controller shall select the signal sighting chairperson ensuring that the person s previous experience indicates that the person is capable of resolving the anticipated complexities of the planned work. The infrastructure controller shall maintain records of the signal sighting chairpersons previous experience. The essence of this section is to ensure that the signal sighting committee includes: a) members who truly represent all interested parties b) members with all the necessary experience and expertise to identify the risks and options available c) members able to make a judgement as to the most suitable measures to propose. Engineering input from non-signalling functions may be needed to identify limitations on the existing infrastructure, to advise on the practicality of modifying the existing infrastructure or to supply information on planned changes to the infrastructure. The representatives of train operators should normally be expected to have expertise on all the known and likely traction units that pass the signal. This may include a large variety of traction types from many train operators. It is quite reasonable for one committee member to represent the expertise on multiple traction types and, by agreement, to represent more than one train operator. It is important to ensure that these members are clear as to the range of traction type and companies they represent. A driver representative may usefully supplement the nominated representative. The representatives of train operators should normally also be expected to have experience of driving the route; however, where the infrastructure is being significantly modified or is new, this experience should be replaced by their judgement based on similar railway environments. It may be necessary to include human factors expertise where specific driver behaviour issues are anticipated or identified as being relevant. The committee member competence process should, however, be designed to provide regular members with sufficient human factors understanding for the majority of situations. The standard identifies the need for competence standards for the chairperson and for certain key members of the signal sighting committee. These competence standards are to ensure that all committees contain a good knowledge of the signal sighting standards, the techniques currently being used to apply them, and above all to ensure a level of consistency nationally. The standard recognises that experience is a key factor, particularly in an activity such as signal sighting where some signals can be far more complex to sight than others. Similarly, the roles of a SPAD investigation require different skills to a new works committee. The standard therefore requires the infrastructure controller to select the chairperson based on their previous experience in signal sighting. This process has the benefit of allowing a chairperson in training to gain experience in the role in a controlled manner. The increased scope for selecting the position and form of new signals requires a greater understanding of the signalling principles compared to the assessment of an existing signal. The chairpersons of committees for new signalling should have a comprehensive knowledge of the content and application of these principles. 18 RAIL SAFETY AND STANDARDS BOARD

21 Page 19 of 71 It should be noted that the competence requirements described are intended to ensure that the key members attending are all familiar with the processes required to be undertaken, together with the technical and human factors that need to be considered. There is no attempt to prescribe competence requirements for the specific areas of expertise brought to the committee by the individual members. Railway Group members should satisfy themselves that those attending have the level of knowledge needed, particular to their specific area. B7.5 Signal sighting committee general competencies B7.5.1 General requirements The infrastructure controller shall ensure that, collectively, the signal sighting committee possesses all of the following: a) all necessary general competencies (see clauses B7.5.2 B7.5.4) b) sufficient local knowledge for the route and signals under consideration c) a working knowledge of the requirements of this standard, and of other associated documents and sources of information (for example, gauge requirements). B7.5.2 Factors for consideration relating to overrun risk The signal sighting committee shall be collectively competent to consider the effects of the following factors on the likelihood of an overrun occurring: a) gradients and permissible/attainable speeds b) aspect sequences and form of junction signalling, including those of existing signals c) routes by which a driver could approach a signal d) grouping of parallel signals and the potential for drivers to be confused as to which one applies to the line they are on e) train working arrangements (including running and shunting movements, and movements onto occupied lines) f) normal stopping positions of trains g) train dispatch arrangements h) complexity of infrastructure (for example, station areas, overhead line equipment, curved approaches, bridges) i) types of trains authorised or planned to operate over the route and in particular their cab sightlines (see also GM/RT2161) j) irregular or excessive spacings of signals k) potential driver distractions (for example, station stops, speed restrictions, neutral sections) l) ambient lighting conditions (including effects of reflected or background sunlight) m) effects of artificial lighting (reflected and background) n) reading through to other signals o) signals unusually positioned (for example, on right-hand side of line) RAIL SAFETY AND STANDARDS BOARD 19

22 Page 20 of 71 p) position of AWS in relation to signal (see GE/RT8035) q) train radio coverage. B7.5.3 Other issues for consideration The signal sighting committee shall be collectively competent to consider the following issues: a) structure gauge, clearances and other matters related to the infrastructure and proposed signal structures, including any alterations to the infrastructure that are planned to take place b) the compatibility of signals and electric traction equipment, for example any conflicts between the positions of neutral sections and the positions of signals (see GK/RT0032), and potential interruptions to signal visibility c) local problems or special conditions, for example a previous history of overruns in the vicinity; driver s complaints regarding the approach view; areas of high vandalism; signs with the potential to obstruct signal visibility. B7.5.4 The choice, position and visibility of new or modified signals The signal sighting committee shall be collectively competent to make recommendations concerning: a) form of new or modified signals b) longitudinal and lateral position of each signal c) the reading time for each signal (including the acceptability of any interruptions) d) alignment and close-up viewing requirements for each signal e) measures for overrun risk control f) train dispatch arrangements from platforms and associated train dispatch equipment (see section C6) g) arrangements for maintainability of signals. The standard requires the signal sighting committee to be collectively competent in a range of activities. There are, however, no specific competence standards that cover this range of activities and therefore a certain amount of judgement is needed in order to satisfy this requirement. Whilst clauses B7.5.1 B7.5.3 are appropriate to all signal sighting committee activities, clause B7.5.4 is appropriate only to new or modified signals. B7.6 Checking and approval of committee recommendations Sighting recommendations for new, modified or repositioned signals shall be checked by an individual competent to act as a signal design engineer. It is not permissible for the recommendations to be checked by a member of the committee that produced them. Assessments of an existing signal, subject to a linespeed increase shall be similarly checked even if no changes are recommended. The sighting details, as recommended by the signal sighting committee, shall be approved by the infrastructure controller when satisfied that the recommendations are adequate to control the risk of an overrun by poor sighting (SPAD). 20 RAIL SAFETY AND STANDARDS BOARD

23 Page 21 of 71 The infrastructure controller s approval process shall review and assess all proposed departures from the requirements of Part C of this standard (see section B6). Where the infrastructure controller does not approve the recommendations, the signal sighting committee shall be reconvened to review its previous proposals, in the light of the reasons for non-approval, and make appropriate alternative recommendations. The infrastructure controller is not permitted to approve any sighting details unless they are supported by the sighting committee (as indicated by the signature of the members see section B7.2). The infrastructure controller shall ensure that the checker has signed the sighting details before giving written approval for the recommendations produced. Details of any departures from Part C of this standard shall be clearly identified and kept with the recommendations. Signal sighting details shall be approved before the production of dependent engineering details commences (except as permitted by GK/RT0207). As a critical part of the signalling design, it is important that signal sighting recommendations are checked in a similar manner to any other design drawings. The role of the checker is to review the forms and comments made by the signal sighting committee, looking in particular for evidence that: a) the assessment processes have been carried out b) the calculations are correct and any assumption made are reasonable c) the recommendation is either compliant or that departures/derogations are identified d) any departure/derogation proposed is adequately justified and supported by evidence, where necessary. To aid consistency in the appearance and positioning of signals, it is normally recommended that all signals in a scheme or locality be assessed by the same committee. Where this is not possible, ensuring that the same checker is used for all signals can help to maintain a level of consistency. It is desirable, but not mandated, that the checker is competent to be a member of a signal sighting committee. As owner of the infrastructure, the infrastructure controller has the power to not approve the recommendation of the signal sighting committee. Whilst Railway Group Standards cannot mandate agreement, this clause directs further attempts to submit modified recommendations. If approval cannot be achieved the infrastructure controller should consider the option of a derogation against clause B7.6. B7.7 Review of overrun risk assessment Where the form or position of a signal, as proposed by the designers, is subsequently altered as a result of the signal sighting committee s recommendations, the relevant parts of the overrun risk assessment (see GI/RT7006) shall be reviewed to ensure that the assessment is still valid. When an overrun risk assessment is carried out, certain details regarding the position of the signal (particularly the longitudinal position relative to points of conflict) are taken into account. If, subsequent to this assessment, these details are altered, it may be necessary to consider how the changes will affect the risk assessment model. RAIL SAFETY AND STANDARDS BOARD 21

24 Page 22 of 71 It should not normally be necessary to completely reassess the signal but, as a minimum, a person with suitable experience in the overrun risk assessment model should be consulted to establish if the changes have increased or decreased the risk. B8 Commissioning checks B8.1 Pre-commissioning checks The requirements for the testing and commissioning of signals are set out in GK/RT0209. The infrastructure controller shall ensure that the tests include: a) a compliance check of the installed signal against the approved sighting details b) an adequacy check to ensure that the driver s view is adequate to reliably identify and observe the signal and correctly interpret the signalling information. The adequacy check shall be carried out from the driving cab using rolling stock appropriate to the line of route or from the track using a periscope or equivalent device. If it is not reasonably practicable to observe every aspect combination from all possible approaches; a representative sample of all combinations shall be observed, selecting, where possible, for this sample, those giving the greatest potential for misreading or incorrect driver response. Where reasonably practicable, these checks shall be conducted before the signals are brought into service. Where this is not reasonably practicable, it is permissible for the checks to be carried out (or completed) within seven days after the signals are brought into service. The compliance check of the signal is to ensure that the installed signal is as designed. It should include checking that the longitudinal and lateral dimensions are correct, that the form of the signal is as shown on the sighting form, the alignment is as specified and the signal is readable from the stated distance. The adequacy check is more subjective, since its function is to confirm that the signal is fit for purpose. Getting a driver s eye view of the signal is therefore essential. The best way to achieve this is from the cab of a train, but where this is not practical, trackside observations with a periscope may suffice. For larger commissionings, time and resources should be allocated to the adequacy checks before the signals are brought into service. For smaller schemes it may be acceptable to carry out some trackside observations (particularly of signals with departures from Part C of the standard) and to use service trains to complete the checks. The standard allows up to seven days in recognition that some lines may have very infrequent train services. Normally, checks should be completed using the first suitable service trains. Adequacy checks should ideally be carried out by the signal sighting committee chairperson; train operator representatives should not be discouraged from being involved at this stage. B8.2 Post-commissioning checks The infrastructure controller and train operators shall ensure that train drivers are consulted and thus given the opportunity to identify any problems or concerns about the sighting of signals that have become apparent by day or night after the introduction of the new or altered signals. This consultation shall be completed no later than three months after commissioning. Any remedial actions identified between bringing the signals into service and the driver s review shall not be delayed awaiting the results of the review. 22 RAIL SAFETY AND STANDARDS BOARD

25 Page 23 of 71 Any remedial actions shall be implemented on a timescale agreed by the infrastructure controller and train operator(s), commensurate with the risks. Where necessary, special measures shall be applied to control the risk pending the implementation of the remedial actions. The general reporting of deficiencies in signal sighting is set out in GE/RT8034. Train drivers should be given the opportunity to comment on the new or modified signals introduced. This process should recognise that problems do not always become apparent immediately but may be dependent on factors such as the time of day, time of year or service patterns. Despite the fact that problems could remain hidden for a long time, the standard requires the consultation to be complete within three months. This is intended to ensure that most issues are dealt with promptly. Existing processes such as SPAD RAM meetings can be used to co-ordinate driver feedback of issues on signal sighting. These would normally be an acceptable means of complying with clause B8.2 of the standard. B9 Records B9.1 Information to be retained A record (also called the signal file) shall be created for each signal. The infrastructure controller shall ensure that, as a minimum, it contains: a) all the current approved sighting details, together with details of the modification history of the signal, including any changes or additions that affect its position, appearance, alignment point or light output b) a record of the approval by the infrastructure controller (see clause B7.6) c) details of assessments, calculations, assumptions and justifications associated with the chosen form, position and alignment of the signal d) reports of problems with the sighting of signals, including those raised at the post-commissioning consultation (see clause B8.2) e) dated photographs of the signal at commissioning and after any alterations to the form or position of the signal. As a minimum, two views of each signal shall be required, one from alongside the most restrictive aspect looking back along the approach to the signal, and a close-range view of the signal, showing clearly the arrangement of the signal elements and signal structure f) details (for example, in diagrammatic form) of any partial or complete obstructions to the driver s viewing of signals that occur within the required reading distance g) details of any derogations, temporary non-compliances or temporary noncompliances pending standards change relating to Railway Group Standards h) details of any departures from the requirements of Part C of this standard i) all ongoing risk assessments involving the signal. The information required in a), c), f) and h) shall be recorded in a standard format approved by the infrastructure controller. GI/RT7001 sets out the general requirements for infrastructure records. RAIL SAFETY AND STANDARDS BOARD 23

26 Page 24 of 71 B9.2 Availability of information The infrastructure controller shall ensure that records of signals be made accessible for the purposes of: a) investigating sighting complaints (see GI/RT7006 or GE/RT8034) b) overrun investigations (see GO/RT3252) 24 RAIL SAFETY AND STANDARDS BOARD

27 Page 25 of 71 c) planning alterations to signalling or linespeed (see GI/RT7006) d) maintenance and visibility checks (see GE/RT8034). When an existing signal is to be modified or re-assessed as part of a linespeed increase or SPAD investigation, it is of considerable help to have access to the original signal sighting details. This is particularly true where the signal appears not to be positioned or aligned as expected. Details such as the identified risks will help to explain any non-standard features about the signal. Over a period of time these identified risks may change and reasons for apparently illogical features may not be obvious unless properly recorded. The information to be retained (clause B9.1) should form a comprehensive record of each signal. Elsewhere in it is referred to as the signal file, but it may also be helpful to think of it as representing part of the health and safety file referred to in the CDM regulations. Photographs taken at the time of commissioning can also provide useful evidence of how vegetation or man-made features have changed from the time when the signal was assessed. It is essential that the reproduction quality of photographs is adequate to see details such as the form of the signal and the condition of lineside vegetation. The best quality is currently obtained using a chemical film with a film speed of no greater than 200 ASA. Digital cameras are, however, rapidly improving and adequate quality may be achieved with a camera using at least 2 Megapixel resolution. It is recommended that the focal length is recorded so that any foreshortening effect of a zoom lens is identified. All these records should be held securely by the infrastructure controller, preferably in a recognised records storage system such as that provided by the Signalling Records Group. It is recommended that the signal sighting form is considered as design detail and included in the design records such that a copy is located in the location case near the signal. Whilst these records should be held securely, they should also be accessible by those needing the information they contain. The standard identifies four activities which should have reasonable access. Appendix 2 identifies a recommended set of forms that can be used to record the details required in clauses a), c), f) and h) of clause B9.1. RAIL SAFETY AND STANDARDS BOARD 25

28 Page 26 of 71 C1 General requirements for positioning and visibility of signals Part C There are two basic roles that a signal can fulfil. Firstly it can act as a stop signal, indicating the end of a movement authority; secondly it can act as a distant signal, warning of another signal ahead that is serving as a stop signal. In many cases a signal will act both as stop and distant signal and therefore the assessment of signal sighting requirements appropriate to that signal should consider both functions. The following identify three features for consideration where stop and distant signals can be treated differently. Approach speed: the first warning or distant signal should always be assumed as being approached at the maximum permissible speed, whereas a stop signal can be assumed to be approached at a speed consistent with the braking curve of a train intending to stop. Long-range/short-range viewing: The driver s requirement for viewing a distant signal is primarily at long range so as to give good warning of the action to be taken and with time to re-check the aspect information, if necessary. As the driver gets closer to the signal (still travelling at the permissible speed) attention may tend to be towards features beyond the signal. Some interruptions to the view are normally acceptable. The driver s requirement for viewing a stop signal is primarily at close range to permit accurate control of the train speed to a stop. Long-range viewing is not essential if it can be assumed that route knowledge and the spacing of the previous signals are suitable to help the driver to judge where the stop signal is. Overbraked or irregular signal spacing may indicate that long-range viewing of the stop signal is necessary. Lateral signal positioning: At long-range viewing, the lateral position of a signal relative to the rails is largely indeterminate (its relation to other parallel signals, however, is most significant). A signal acting only as a distant signal can be positioned laterally with rather more freedom than a signal which acts as a stop signal, so long as there is no confusion as to which line it applies to. The ideal location for a signal to be seen at close range is as close to the driver s line of sight as possible (see Appendix 3) so as to remain a prominent feature in the field of view. C1.1 Longitudinal positioning of signals C1.1.1 Signals on parallel lines Where lines running parallel to each other are signalled in the same direction and drivers on one line can see the signal(s) on the parallel line(s), the signals for each line shall normally be placed so as to be at the same longitudinal positions as those on the parallel lines. This is called parallel positioning of signals. Signals shall be regarded as parallel if they are within 20 m of each other longitudinally. Where lines running parallel to each other are signalled in the same direction but signals are not provided on each line, particular consideration shall be given to ensure that the risk of driver confusion or misread is minimised. Where lines that are operated as separate routes run parallel (for example, the DC lines alongside the West Coast Main Line, or LUL lines adjacent to Network Rail controlled infrastructure), this section applies to each route individually, and not to the set of parallel routes. Nevertheless, consideration shall be given to the 26 RAIL SAFETY AND STANDARDS BOARD

29 Page 27 of 71 risk of drivers on either route erroneously obeying signals that apply to the adjacent route. A departure from the requirement for signals to be positioned to appear in parallel shall demonstrate, as a minimum, that: a) during the approach to the signals, the driver is unlikely to be confused as to the position of the signals or which signal to obey. In particular, the crossing-over effect (in which the relative positions of the signals appears to change during the approach) shall not occur b) the overrun risks associated with the non-parallel positions are acceptably low. The principal purpose of positioning signals in parallel is to help the driver to identify which signal applies to which line and to ensure that the distance to the next signal is not dependent on which route is taken. specifies a maximum longitudinal distance that the signals may be out of parallel. In practice it is often more important to ensure that the signals appear parallel to an approaching train, particularly the long-range view up to approximately 200 m. It is during this part of the approach when it is most critical that the driver identifies the correct signal to obey. If the signals appear in the wrong order as a result of non-parallel signals and the cross-over effect, there is a significant risk of a driver misreading which signal to obey. See also Appendix 3. When the driver is within approximately 200 m of the signal, it should be possible for a driver to focus on a single signal and, if appropriate, control the train speed to a stop in the correct position. Where permissible speeds on parallel lines are not equal it can be difficult to keep signals parallel and meet the signal spacing requirements of GK/RT0034. The overbraking permitted by GK/RT0034 allows some flexibility, which may be sufficient to keep the signals parallel, albeit with some loss of headway on the slower line. Alternatively, it may be possible to meet the requirement for parallel signals and the requirements of GK/RT0034 by providing three aspect signalling on the slower line and four aspect signalling on the higher speed line. Whilst this is strictly an issue for the scheme design stage rather than the signal sighting stage, it serves as an example of how the selection of the signal position is a multi-stage process. Locations where signals from other infrastructure controllers (such as LUL), or even from roadways, are positioned alongside Network Rail controlled infrastructure can create additional problems. Railway Group Standards are only applicable to Network Rail controlled infrastructure and therefore it is not possible to mandate responsibilities elsewhere. Any co-operation required to resolve signal sighting issues should be achieved by means of consultation and negotiation. Occasionally it may be considered preferable to not position signals in parallel. Situations such as exit signals from loops or sidings are generally considered acceptable so long as consideration is given to the sequence observed by a driver approaching on the neighbouring running line. Techniques suitable for helping to differentiate a loop/siding signal from running signals include a lower vertical location, aligning the loop/siding signal away from the running line or use of a spreadlight lens. Non-parallel running signals should occur only in very specific circumstances. GK/RT0032 permits this on reduced capacity bi-directional lines. Stations may require non-parallel positioned starting signals, mid-platform signals or closing-up signals. Examples also exist where fast line signals are spaced at half the RAIL SAFETY AND STANDARDS BOARD 27

30 Page 28 of 71 frequency of slow line signals. All occasions of non-parallel signals should be treated as departures and processed in accordance with clause B6. C1.1.2 Train radio coverage Consideration shall be given to the adequacy of train radio coverage when positioning signals. If necessary, the coverage shall be improved to suit the positioning of the signal. Where the radio system automatically routes calls to the appropriate signaller, the radio system shall be checked to ensure that routing is applied correctly (see GE/RT8080). It is always desirable for there to be radio coverage at locations where a train might stop, which includes signals. The NRN system was never designed to cover 100% of the railway. In particular, tunnels and deep cuttings will not normally be covered. In such circumstances it may not be possible to position signals, so that radio coverage can be provided. In CSR areas the radio system coverage is nominally 100% but small pockets of poor or no reception may exist. Local knowledge may be able to identify where these exist and either avoid as signal positions, or arrange for the radio system to be modified to reduce or remove the problem. Although the introduction of GSM-R should bring many improvements, the cellbased routing of calls (see GE/RT8080) may require a check to ensure that calls (particularly emergency calls) can be received by the correct signaller. C1.2 Lateral positioning of signals C1.2.1 General requirements Each signal shall be positioned laterally as follows: a) on the left-hand side of the line as seen in the direction of travel to which the signal applies, except as permitted by clause C1.2.2 b) so that drivers on all lines readily associate the signal with the line to which it applies and not with any other line. C1.2.2 Positioning of signals on right-hand side of line A departure from the requirement for signals to be positioned on the left-hand side of the line shall demonstrate, as a minimum, that: a) the readability is improved for trains approaching the signal and SPAD risk is reduced b) for stop signals, the signal is readable by drivers of trains stationary in the normal stopping position at the signal (clause only applicable to stop signals) c) there is no line immediately to the right of the signal, such that drivers on either line might associate the signal with that line rather than with the line to which it actually applies (signals on lines separated by an island platform can usually be regarded as meeting this criterion). For close-range viewing and when stationary at the signal, the best lateral position for a signal is to the left-hand side of the track and as close to the track as is reasonably practicable. For long-range viewing, such a location may not be suitable because of obstructions such as platform buildings or the side of a cutting on a left-hand curve. A right-hand signal may appear to provide a better option for viewing the signal throughout the required reading distance and possibly avoid the need for a co- 28 RAIL SAFETY AND STANDARDS BOARD

31 Page 29 of 71 actor or banner repeater signal. The signal sighting committee need to be satisfied that a driver will recognise a right-hand signal as the signal that is to be obeyed, and equally that drivers on other lines should not wrongly obey the signal. If the signal acts as a stop signal the committee should ensure that the driver of a train standing at the signal still has a sufficiently prominent view of the signal. Where platforms are on the right-hand side, consideration should also be given to the ability of platform staff observing the signal aspect. The two most likely options would be a right-hand signal and a train stop marker sufficiently far from the signal to give the driver an acceptable view, or a left-hand signal plus OFF indicator. Further requirements and guidance on platform starting signals is provided in section C6. On lines with only one track signalled in the direction being considered, the likelihood of the signal being associated with the wrong track is low and therefore right-hand signals may be suitable to improve reading time. Distant, co-acting and banner repeater signals where a close-range view is not critical could possibly be positioned further from the running line or even to the right of a two-track formation. On two track railways with bi-directional signalling, it is common practice to place the wrong road signals on the right-hand side; however, these should always be positioned with particular regard to any need for close-range viewing or where the approach makes observance of both signals difficult, such as after a bend. On multi-track railways right-hand signals are much more likely to be mis-read as applying to the adjacent track and therefore it is less likely to be suitable unless there is some physical separation to help relate the signal to the correct track. Platform starting signals at multi-track stations may be acceptable positioned on the platform side. C1.3 Visibility of signals C1.3.1 Determining the required reading distance Where the achievable reading distance (see clause B5.5) is initially assessed as less than the minimum reading distance (see clause B5.4), then mitigating measures shall be considered in order to increase the reading time available or to reduce the minimum required. Consideration shall be given to hazards generated by the introduction of the mitigating measures. Where the hazards introduced are considered more significant than the improvements to reading distance generated, the mitigating measure shall not be introduced and other options shall be developed. A departure from this requirement to permit an achievable reading distance less than the minimum reading distance, shall demonstrate, as a minimum, that: a) it is not reasonably practicable to increase the achievable reading distance b) the difference between that which is achievable and the minimum reading distance is tolerable. When an acceptable achievable reading distance has been determined this shall be recorded as the required reading distance. (GE/RT8034 sets out requirements to maintain the required reading distance for the life of the signal.) Sections B5.2 to B5.4 require an assessment to be made of the minimum time that should be provided to the train driver in order that the signal is observed correctly in the vast majority of situations. Clause B5.5 requires an assessment of what can be achieved on site and calls for reasonably practicable efforts to maximise this. RAIL SAFETY AND STANDARDS BOARD 29

32 Page 30 of 71 Clause C1.3.1 requires the two to be compared. If the achievable distance is greater than the assessed minimum, the task is complete and the achievable distance is recorded as the required reading distance. GE/RT8034 is designed to ensure that this reading distance is then maintained. The more difficult task occurs if the achievable reading distance is less than the assessed minimum. In this case a range of options should be considered, the most likely of which are listed in Appendix 1, with the purpose of trying to find an achievable reading distance that can at least be considered acceptable given the many physical and financial constraints imposed and the consequences resulting from a driver failing to assimilate the signalling information correctly. None of the options listed are ideal, each has problems associated with it. It is the difficult job of the signal sighting committee to judge how best to resolve the problem. Appendix 1 contains tables that identify key factors (positive and negative) associated with the likely options which should be considered. When a departure from the requirement of clause C1.3.1 is being considered, the justification will firstly need to demonstrate that, for each option considered, there was either no benefit or that the negative factors outweighed the positive factors (see clause C1.3.1a)) and, secondly, demonstrate that the difference in achievable and minimum reading distance can be tolerated (see clause C1.3.1b)). An absolute minimum has not been specified in the standard, firstly, because there are many factors that make each circumstance individual and, secondly, because there is inevitably a tendency to take any stated value as an acceptable answer. Factors that are critical to the justification include: a) whether the signal protects a conflicting move (junction signal) b) whether the signal acts as a first caution to a junction signal c) any excess braking distance to the stop signal when the signal being assessed is acting as the first caution d) any other departures from Part C at the signal being assessed or those within a braking sequence, including the assessed signal. A human factors assessment of the task of reading a signal (cognitive task analysis) may be used to demonstrate acceptable reductions in reading time. The reference section in this Guidance Note identifies one such assessment. It is recommended that as an aid to consistency, the infrastructure controller should determine national guidelines for circumstances where a departure from this clause is required (see guidance to clause B6). C1.3.2 Close-up viewing of the signal For signals capable of displaying a stop aspect, all elements of the signal shall be readable from the driving cab with no interruption of view between 40 m from the signal and the closest point at which a driver is expected to bring his train to a stand at the signal at danger. It is permissible to apply other measures, such as described in section C5.2, in order to meet this requirement. Where the signal is fitted with a device that offers a means of enhanced close-up viewing (for example a close up viewing segment) the device shall be orientated to optimise the readability over this distance. Where planned stopping points are to be closer than 15 m before the signal, a vision plot, identifying the limitations caused by cab sightlines of the relevant types of rolling stock, shall be carried out. 30 RAIL SAFETY AND STANDARDS BOARD

33 Page 31 of 71 Section C6 sets out the special requirements for platform starting signals. It is essential that the signal aspects are readable when a train is brought to a stand in front of a signal. There are a series of factors that can affect this: a) the lateral position of the signal b) the optical design of the signal c) restrictions of view caused by the cab design d) how close to the signal the train can be expected to stop. All four of these factors are interrelated. For example, a signal positioned laterally close in to the driver s eye should make it easier for a driver in a cab with limited sight lines approaching the signal to continue to view the signal. Similarly, the closest stopping point in front of the signal directly affects the required optical performance of the signal, as does the lateral position of the signal. Main colourlight signals are a particular case where the equipment is designed to optimise the long-range view resulting in the close-range view being less good. As a result, main colourlight signals normally provide a means of enhanced close-up viewing (for example a hot strip ) to improve the readability at close range. A hot strip is not the only option of providing good close-up viewing, a co-acting signal could be provided for this purpose which could take the form of a miniature unit designed purely for close-up viewing which can then be angled directly towards a stationary train. Minimum requirements for cab sightlines are set out in GM/RT2161 but the existing range of cab designs exhibit a considerable range of sightline capabilities. In particular, a centre gangway can severely restrict the view of righthand positioned signals. Defining the closest stopping position for a train at a signal is not a straightforward process. Existing driving policies encourage drivers to stop between 15 m and 25 m back from the signal. At these distances, there is rarely any difficulty in viewing the signal; however, these target stopping positions have been developed to allow for minor misjudgements in the train braking, and so it is reasonable to expect a few trains to stop closer than these policies suggest. Technically, a train could legitimately stop in line with the signal but it is not practical either for the signal to be expected to be readable from this position or for the cab sightlines to cater for this. The following strategy may provide a suitable balance between practicality and theoretical need: C B A Limit of close viewing of the signal (infrastructure controller defined) Limit of cab sightlines (GM/RT2161 refers) Planned Stopping Point Figure 2 Close up viewing of the signal (Clause C1.3.2 in GE/GN8037) RAIL SAFETY AND STANDARDS BOARD 31

34 Page 32 of 71 a) At the planned stopping point (A on Figure 2) the driver should always be able to view the signal from the normal driving position. b) If the train stops between points A and B the driver should still be able to view the signal from the normal driving position. c) If the train stops between points B and C the driver should be able to view the signal but may need to lean forward, or stand, in order to achieve this to overcome limitations imposed by the cab design. d) If the train stops closer than point C the driver may not be able to view the signal from the cab and may need to get out of the train to view the signal. At some signals such as platform starter signals, the planned stopping position may be considerably closer to the signal than normal. It is essential that the signal is not only visible from the normal driving position but is sufficiently prominent to minimise the risk of a SAS SPAD prompted by a right away indication being received in error. Similar problems may be encountered where trains reverse leaving the driving cab close up to the signal. See also Appendix 3. A vision plot for a particular style of cab should identify how close the vehicle can come to a signal before the cab structure obscures the driver s view. It will be necessary to assess this for a range of signal positions. Appendix 4 outlines the vision plot process. C1.3.3 Visibility of signals on other lines So far as is reasonably practicable, signals shall be positioned and aligned so as not to cause confusion in the correct observation and interpretation of other signals by drivers on other lines, whilst ensuring that a driver has adequate visibility of signals on their line. On parallel lines signalled in the same direction, consideration shall be given to the provision of a means by which a driver can more readily identify the signal that is applicable to their train. This is particularly important on lines with three or more parallel signals, or where a junction is unusually complex. There are several scenarios where a committee should consider the possibility of a driver selecting an incorrect signal to obey. On a multi-track railway with two signals in parallel there should not be much problem so long as basic good practice is applied such as keeping signals parallel and, where possible, using consistent structures on successive signal sections (see clause C1.4). Problems may be introduced where an additional signal is introduced such as for the exit from a siding or loop. If this coincides with a curved approach which stops all signals being seen together, there is a risk of mis-reading. Solutions to this problem could include the use of a spreadlight lens in the loop/exit signal, align it away from trains on the main line or to position the head significantly lower than the others. GK/RT0032 permits the use of a shunt signal in some circumstances. GK/RT0060 permits the use of approach lit signals where this would assist the driver in identifying the correct signal. On a multi-track railway with many parallel signals, there is likely to be concern about a driver s ability to identify the correct signal. Line identifier plates have been used (see clause C7.5) in several complex locations to provide assistance to the driver. Selecting the position of these plates needs great care and should be done on a site-specific basis. The main objective of the positioning should be to ensure the correct association of signal with track. At London Euston where line identifiers have been introduced, an additional feature intended to assist drivers has been to provide an alphanumeric route indicator on every signal such that a driver observes the route indicator and then 32 RAIL SAFETY AND STANDARDS BOARD

35 Page 33 of 71 searches the next signal gantry for that line identifier. It should be noted, however, that the individuality of the more complex parts of the railway network means that strategies used at one location may not be suitable for other sites. Staggering the height of signals on a gantry or the height of line identifier plates on a gantry may be considered as a means of helping to identify the correct signal. The benefits from this approach are not easily predictable and it is recommended that a virtual reality model is used to assess the effect before introduction onto the railway. C1.3.4 Read-through Consideration shall be given to the risk of a driver reading-through to a signal beyond that which should next be obeyed and which might reasonably be expected to be displaying a less restrictive aspect (typically beyond a junction or level crossing). Where this risk is considered unacceptable, measures shall be introduced (such as reducing the conspicuity of the forward signal or applying special controls to the signal aspect) to mitigate the risk (see GK/RT0060). There are several sections of the railway network where a driver is able to see two or more signal sections ahead because the railway is straight. In itself this does not present any significant risk, indeed it means that the driver is always able to optimise the train speed to suit the signal aspects ahead. There are, however, some circumstances when read-through can create undesirable hazards. The most common read-through problem exists where a driver can see beyond the nearest signal to a less restrictive aspect, which in some circumstances takes precedence in the driver s mind; typically this will be an automatic signal beyond a junction or level crossing. The risk of read-through will increase if the near signal is obscured while the far signal is still visible. If such a situation appears possible, the signal sighting committee should consider ways to reduce the prominence of the far signal, increase the prominence of the near signal or, if necessary, call for special controls to be applied to hold the far signal at red. Stopping positions very close to a signal (typically on a platform) can increase the risk of the driver concentrating on the far signal because the near signal is out of the central vision area (see also clause C6.4 and Appendix 3). On straight lines where several signal sections are visible to the driver, a signal can become obscured (typically by OLE masts) such that the driver can see the near signal, not see the next, but is able to see one or more after that. This effect has been known to cause problems and should be avoided, if possible. C1.4 Consistency of form C1.4.1 Form of signals Successive signals for the same direction of traffic flow shall be reasonably consistent in form, in order to facilitate correct observation by the driver of the information presented. Consideration shall be given to the form of new, modified and existing unaltered signals preceding/following the new or modified signals. The following factors shall be considered: a) the relative position of elements of signals b) the choice of route indicator types c) the lateral position of signals RAIL SAFETY AND STANDARDS BOARD 33

36 Page 34 of 71 d) the relative position of parallel signals. Colourlight and semaphore elements shall not be intermixed on a signal, except that it is permissible to use alphanumeric route indicators in conjunction with semaphore stop signals. A departure from this requirement to permit inconsistent signal forms shall demonstrate, as a minimum, that: e) the resulting signals do not introduce any foreseeable driver error f) the inconsistency has been introduced to improve drivers understanding. C1.4.2 Form of structures Consideration shall be given to the advantages, from a driver s perspective, of consistency in the type of structures on which successive parallel signals are mounted. The inconsistent use of gantries, cantilevers and straight posts for successive groups of parallel signals can contribute to the misreading of signals by drivers. Semaphore signals are notoriously inconsistent in form and lateral positioning, with many drivers claiming that this helps to identify them individually., however, requires consistency of form, which appears to contradict existing experience. With the introduction of colourlight signals and track circuit block sections, a driver is no longer able to identify each signal simply by its shape and form. It is therefore necessary to keep the appearance of signals and their structures as consistent as possible so that a driver can reinforce the observance of signals with recognition of a familiar form. On a typical four-track railway, the consistency of the parallel signals is particularly important. Although gantry structures are almost certain to be needed, it is recommended that the signal applying to the track on the edge of the track formation should be positioned with the red 3.3 m above rail level such that there is a stagger between the heights of the two signals. The deliberate introduction of something different can help to highlight the signal as being special in some way. This approach has been used to identify multi- SPAD signals. Clearly, if applied too frequently the effect is destroyed. C1.5 Lineside signs in the vicinity of signals Lineside signs, required to be observed by a driver for the purpose of controlling the train, shall be positioned so as not to create unnecessary distraction from the driver s primary role of observing signals. So far as is reasonably practicable, signs shall not be positioned between the signal and its associated AWS or within 50 m beyond the signal. If the sign has AWS equipment associated with it, the sign shall be positioned such that the AWS for the sign does not fall between the signal and its AWS equipment. Where this is not achievable, the sign shall be positioned at the signal and consideration shall be given to the size, lateral position and reflective properties, to ensure that all the information displayed is proportionate and that no part is so prominent as to make other parts appear insignificant. GK/RT0038 sets out requirements for positioning signs associated with permissible speeds. GE/RT8035 sets out requirements for the positioning of AWS track equipment. There are a considerable amount of lineside signs required to be fitted, many of these are to provide instruction to the driver about controlling the train s speed, 34 RAIL SAFETY AND STANDARDS BOARD

37 Page 35 of 71 traction system, radio etc. Whilst the driver s route knowledge should ensure that these signs act only as reminders, they still represent a source of distraction from observing signal information. Wherever possible, these signs should be positioned away from signals such that they do not create a distraction from the primary need to observe the signal. Defining the size of this excluded zone is problematic and not currently supported by any formal evidence. The quoted details in the standard represent best practice, and a simple rule that fits logically with existing rules for temporary and emergency warning boards, and ensures that any AWS warnings received will be associated with the correct sign or signal. It will not always be possible to position signs away from signals, therefore an alternative position is to place the sign alongside the signal. In this case the presentation of all the information should be considered to ensure that all necessary information can be processed. In particular, it is believed that it is better to position signs and signal heads close together and avoid the need for a driver to have to look in multiple directions to find all the information. Therefore, signs should be fixed to the signal post or alongside the signal head, particularly where signals are mounted on gantry structures. The standard size (900 mm diameter) of many signs when mounted alongside a signal head appears out of proportion and a smaller size should be considered in order to keep the prominence of the sign in proportion with the importance of its information. Consideration should also be given to the reflective material used. Signs fitted above driver s eyelevel will be at risk from glare reflected from the sun and, at least in theory, are no longer in the beam of the train s headlight. C2 Colourlight main signals and associated route indicators C2.1 Form of signals C2.1.1 Main colourlight signals Main colourlight signal heads comprising more than one element shall usually be mounted vertically. Where necessary, for visibility purposes or because of constraints on space, it is permissible to mount the elements other than in a vertical array, provided that any double yellow aspect, and any flashing double yellow aspect, continues to be displayed vertically. If more than one element is used to display aspects, the order of proximity of lights to the driver s eye as he passes the signal shall be as follows: a) red aspect light closest b) first yellow aspect light c) green aspect light d) second yellow aspect light (required only for double yellow aspect and flashing double yellow aspect). Where any of the four elements listed above are not provided, or are combined into a single unit, the order described above shall be maintained by the remaining elements. Exceptionally, it is permissible for a signal required to show red, yellow and double yellow only, to have the red light separating the two yellows, but only if the RAIL SAFETY AND STANDARDS BOARD 35

38 Page 36 of 71 red aspect remains near driver s eye level so that the sighting of the aspect cannot be obscured by signal hoods. The familiar form of a main colourlight signal is of a vertically arranged multiple element signal with the red as the lowest aspect. Given the familiarity of this form, consideration should be given to possible uncertainty generated if other arrangements are introduced. There are, however, situations where a horizontally mounted signal head has permitted the fitment of a signal in confined spaces where a more conventional form would not. More recent introductions of colourlight signals where one element can produce a range of aspects, provides opportunities to fit main colourlight signals into a physically smaller space. If this form of signal is used, consideration should be given to the spacing between the elements such that a double yellow aspect appears in a normal manner to the driver. C2.1.2 Positioning of route indicators Position light junction indicators (PLJIs) shall usually be positioned with the pivot light directly above the main aspects. It is permissible for the indicators to be positioned immediately to the side of the main signal (left or right), but only where there is no other way of ensuring adequate readability. Where a PLJI is positioned to the side of the main signal, the pivot light shall be positioned adjacent to the red aspect. Position indicators 1, 2 and 3 shall be placed only to the left of the main signal, and position indicators 4, 5 and 6 only to the right (see GK/RT0031 for details of position notation). It is permissible for alphanumeric route indicators to be positioned either above, or immediately to the side of the main signal, whichever gives the optimum readability. Where an alphanumeric route indicator is positioned to the side of the main signal, it shall be positioned to the left of the main signal if the signal is on the left-hand side of the line, and to the right of the main signal if the signal is to the right-hand side of the line. No guidance is provided. C2.2 Alignment of signal The signal shall be aligned so as to optimise the readability of the signal by the driver of an approaching train, taking into account the need for all illuminated elements of the signal to: a) be readable at the start of the required reading distance b) remain readable throughout the required reading distance (except during those interruptions to the view considered acceptable by the process set out in clause B5.3). For most signals on the network a simple set of rules to determine the alignment should suffice. There will be some circumstances where great care is needed to determine the alignment of the signal. Particular care will be needed on tight curves where the width of the beam may not be able to cover the required reading distance: at signals where trains can approach from different directions, at signals where there is a risk of reading across to the wrong signal (parallel signals with a curved approach may fall into this category) or at signals where a read through risk has been identified. The height of the beam produced from a conventional signal with a filament bulb is considerably narrower than the width of the beam. When the signal is positioned to the side of the track at the recommended height and aligned to point 36 RAIL SAFETY AND STANDARDS BOARD

39 Page 37 of 71 3 m above the rail, the driver will remain in the beam throughout the approach. When the signal is significantly above or below the driver s eye level (for example when fixed on a gantry) the driver may not be in the beam for the whole of the required reading distance. Particular care should be taken therefore to optimise the vertical alignment of signals on gantries or mounted at ground level. Consideration should also be given to providing assistance to the process of setting and checking the alignment. Some form of alignment point marking may be helpful, particularly when the alignment is non-standard. C2.3 Lateral position of signal C2.3.1 Height of signal above rail level The centre of the element that displays the most restrictive element shall be as close as possible to, but not usually less than, 3.3 m above rail level. Where necessary, to provide adequate visibility, it is permissible for the most restrictive element of the signal to be up to 5.1 m above rail level. In deciding whether it is acceptable to use a height greater than 3.3 m, attention shall be given to: a) the difficulty that a driver might have in relating the signal to the line to which it applies (especially where the approach to the signal is curved, with signals on parallel lines) b) the readability of the signal during the driver s approach (see clause C2.2) c) the readability of the signal when the driver is close to it (see clause C1.3.4) d) possible problems with incident or background sunlight, or with the background against which the signal is viewed, preventing the aspect from being discerned e) the risk of reading through to another signal ahead, instead of obeying the correct signal. It is also permissible, where necessary, to provide adequate visibility, because of space constraints, or to assist with the association of signal with line, for a signal to be positioned with some or all of its elements below 3.3 m. In deciding whether it is acceptable to position a signal below the height specified in this section, particular attention shall be given to: f) the readability of the signal during the driver s approach (see clause C2.2) g) lines where the driver s eye level on all trains is lower (for example, LUL stock) h) the readability of the signal when the driver is close to it (see clause C1.3.4) i) risk of obscuration of the signal (for example, by the signal head hoods, signs, persons on the track) j) the presence of a co-acting signal. For close-range viewing of a main colourlight signal the ideal height of the signal head is at driver s eyelevel. There are many reasons why, in practice, this is not the best position. Unfortunately, driver s eyelevel is not consistent. Humans, cabs and seat design all vary making it impractical to define a specific value. Combining this fact with the current use of hoods on signal lenses, which tend to making observing from above difficult, it becomes necessary to ensure that the signal head (and in RAIL SAFETY AND STANDARDS BOARD 37

40 Page 38 of 71 particular the most restrictive aspect) is no lower than the highest expected driver s eye. Consequently a height of 3.3 m is recommended. There are many other reasons why the signal head might need to be higher than 3.3 m. Long-range viewing requirements and structure gauge clearance are two valid reasons for raising the height; however, any increase in height has the potential to create problems, particularly at close range. Signals placed on gantries can be more difficult to associate with the correct track. This is thought to be because there is no post to form a line between the signal head and the track to which it applies (see also the guidance to section C1.4). Sometimes signals need to be positioned lower than normal, for example to improve sighting under an obstruction, to distinguish a signal as applying to a siding or because of particular rolling stock using the line such as the Tyne and Wear metro. The standard identifies issues to be considered to ensure the signal remains readable. C2.3.2 Horizontal displacement of signal relative to rails Signals to the side of the structure gauge (for example, post mounted) shall usually be positioned as close as reasonably practicable to the nearest running rail of the line to which they apply whilst still maintaining clearances as specified in clause B5.5. It is permissible to position signals further away from the nearest running rail if, by doing so, the driver s view is improved. Signals above the structure gauge (for example, gantry or cantilever mounted) shall usually be positioned with the centre-line of the most restrictive aspect approximately 900 mm to the left of the running edge of the left-hand rail. It is permissible for such signals to be positioned further to the right if by doing so: a) the readability throughout the required reading distance is improved, or b) the association of the signal with the correct line is improved. Signals positioned to the right of the track centre-line shall be considered as righthand mounted signals and therefore require a departure from clause C1.2 of this standard. The requirement to keep the signal as close as reasonably practicable to the nearest running rail serves two important processes. First it helps to ensure that the signal is associated with the correct line in the mind of an approaching train driver. Secondly it helps to ensure that the close-up viewing requirements, mandated in clause C1.3.2 are met. Any consideration to positioning the signal further from the running line than usual should consider carefully the potential negative effects on line association and close-up viewing. Association with the correct line becomes much more critical where parallel signals exist, on a simple two-track railway there is normally only minimal risk. Close-up viewing requirements only apply to signals capable of displaying a stop aspect. Further guidance on this subject is provided in clause C1.2. C2.4 Partial failure of a signal aspect Consideration shall be given to the possible effect on the displayed aspect during a partial failure such as a filament failure. Where this is considered to have a noticeable effect on the appearance, position or alignment of the lit aspect, an assessment shall be carried out to determine whether the consequent change is acceptable. 38 RAIL SAFETY AND STANDARDS BOARD

41 Page 39 of 71 The design of signal heads has, up until now, ensured that the appearance of the aspect during partial failure (first filament) is the same as in normal operation. New designs of signal heads can introduce alternative methods of proving a display during partial failure, which may give a different appearance to the driver. Some signals at Manchester are designed to display an aspect out of a different element when a first filament failure occurs. The standard requires this effect to be considered to establish if the resulting display is acceptable to the driver. The more common style of change-over to a second filament in the same signal element can produce a different beam pattern, and in general the effect will be to produce a narrower, taller beam. If the beam alignment is critical to a particular signal (for example on a tight bend) the failure of the first filament may have a significant effect on the readability of the signal. C3 Position light signals (PLS) and associated route indicators C3.1 Form of signals C3.1.1 Subsidiary signals Except as permitted below, subsidiary PLSs shall be positioned immediately below the red main aspect. It is permissible for the subsidiary PLS to be positioned immediately to the left of the main red aspect when the signal is to the left of the line to which it applies, or to the right if the signal is to the right of the line to which it applies, if either: a) route indicators are associated with the PLS, or b) the effect of placing the PLS beneath the red aspect would be to make the red aspect less readable, because of its increased height. Where route indicators are associated with a subsidiary signal, they shall usually be positioned immediately above the PLS. A PLS used as a Proceed on Sight Aspect (PoSA) shall be positioned in accordance with this clause C A departure from the requirements of this clause shall demonstrate that the form of the signal as a whole is improved or that limitations such as the structure gauge make compliance impractical. Rules governing the position of a subsidiary PLS relative to the main aspects have changed many times in the past. It can be unclear how to position a subsidiary PLS and any route indicator to be consistent with existing signals. The overall aim for new or modified signals should be to ensure that the association of route indicator and subsidiary aspect and the association of subsidiary aspect to the main aspect are both reliably made by drivers. C3.1.2 Independent PLS Route indicators associated with an independent PLS shall usually be positioned immediately above the PLS. A departure from the requirements of this clause shall demonstrate that the form of the signal as a whole is improved or that limitations such as the structure gauge make compliance impractical. Where route indicators are required for an independent PLS the usual arrangement is for the indicator to be placed above the PLS. However, the gauge restrictions often imposed on these types of signals may make it impossible to achieve this. RAIL SAFETY AND STANDARDS BOARD 39

42 Page 40 of 71 One technique used where space is very limited is to position the route indicator beyond the PLS such that when the driver is at a stand in front of the signal, the indicator appears to be just above the PLS. Positioning the route indicator below the PLS, although not consistent with usual practice, has the advantages of raising the PLS a little closer to the driver s eye level (see clause C3.3.1 of the guidance) and helps to keep the PLS aspects clean by raising them clear of the ballast. C3.2 Visibility requirements C3.2.1 Required reading distance The required reading distance for an independent PLS shall be sufficient for a driver to brake to a stand at the signal from the permissible speed applicable to the train movement. Additional requirements for a subsidiary PLS used for controlling movements onto an occupied line are set out in GK/RT0044. The requirements in this section are not applicable when an independent PLS acts as a preset shunt signal. C3.2.2 Alignment PLSs and associated indicators shall be aligned towards a train standing at the normal stopping position on the approach side of the signal. The rules for reading time and alignment as applied to main signals are not suited to a PLS. Clause B5.2.1 recognises that drivers approaching a PLS should travel at a speed at which they can stop short of obstruction or stop signal encountered (often referred to as proceed on sight ). Typically, 50 m reading distance of the signal will be acceptable. When used to control a movement onto an occupied line (typically into a platform) special requirements relating to the visibility of the section of line ahead of the signal shall be applied, as set out in GK/RT0044. The alignment of a PLS is not critical because the beam is not focused like a main aspect. Since there is no alignment device on a PLS the alignment can only be approximate. C3.3 Lateral position of independent PLSs C3.3.1 Height of signal above rail level Independent PLSs shall usually be positioned below driver s eye level so as to be clearly differentiated from main signals. The selection of the height shall be determined by: a) limitations of structure gauge b) achieving consistency of height within a junction area c) cab sightline limitations. C3.3.2 Horizontal displacement of signal relative to rails Independent PLSs shall be positioned in accordance with the requirements of clause C Keeping independent PLSs below a driver s eye level is a simple but effective method of separating the functions of a PLS from that of a main signal (normally above driver s eye level). Positioning them literally at ground or rail level can increase the need for cleaning and it is known that cab design can restrict the 40 RAIL SAFETY AND STANDARDS BOARD

43 Page 41 of 71 driver s view of rail level, in some cases to no closer than 50 m. Where practical it may be desirable to raise independent PLSs on a short post (typically 1 m) to improve sighting from the cab, to keep the signal cleaner and to improve maintenance access. C4 Semaphore signals and signs that perform the function of signals C4.1 Form of signals and signs C4.1.1 General requirements In meeting the requirements of this standard, all semaphore signal arms, discs and indicators mounted on a common structure and applying to the same line shall be treated as elements of a single signal. Arms shall be of the upper quadrant type, unless existing signals in the area are wholly or largely lower quadrant. All arms on a structure shall be of the same type. Semaphore shunt signals shall be of the disc type, unless existing shunt signals in the area are wholly or largely miniature arm type. C4.1.2 Signals with multiple arms Where multiple routes exist from a main signal, a separate arm shall be provided for each route, arranged horizontally. The position of each arm from left to right shall correspond to the direction of the route from left to right. Routes which differ in speed shall be stepped in height; the highest arm shall apply to the fastest route and subsequent arms to either side shall be progressively lower. The dolls shall be a minimum of 1830 mm apart and the arms stepped in height by a minimum of 760 mm increments. The following alternative forms of junction signal are permitted if their use improves clarity or consistency: a) Junction signals arranged with multiple routing arms arranged vertically above one another. The position of each arm from top to bottom shall correspond to the direction of the route from left to right. Arms shall be a minimum of 1680 mm apart. This arrangement shall be used only where the speed differential between the fastest and slowest route is no greater than 10 mph and where the permissible speed is no greater than 40 mph. b) Junction signals arranged with a single arm and a standard alphanumeric route indicator positioned directly underneath. Consideration shall be given to the risk of a bright route indicator obscuring a relatively dim signal lamp. Where stop and distant arms applying to the same route are to be mounted on the same post, the distant arm shall be positioned 1680 mm (+/- 200 mm) below the stop arm. Where stop and subsidiary arms applying to the same route are to be mounted on the same post, the subsidiary signals shall be fitted 1070 mm (+/- 200mm) below the arm next above. Where a distant signal is fitted below a stop signal, any subsidiary signal shall be fitted below the distant signal arm. The discs / arms of shunting signals shall be arranged directly above one another and their positions from top to bottom shall correspond to the direction of the route from left to right. Where more than one disc / arm is mounted on a single structure, all such discs / arms shall be of uniform type (that is, all discs or all arms), size and design. Where arms are used, they shall be spaced 1070 mm (+/- 200 mm) apart. C4.1.3 Lighting of semaphore signals RAIL SAFETY AND STANDARDS BOARD 41

44 Page 42 of 71 Except as permitted below, the light intensity of all main semaphore signals shall be reasonably consistent within the area controlled by a signal box. It is permissible for the light intensity of a semaphore signal to be significantly greater than that of other signals controlled by the same signal box, if by doing so it improves the readability of that signal and does not increase the likelihood of drivers misreading or disregarding other signals. Although this section of the standard sets out options for the appearance and layout of semaphore signals, when alterations are to take place, a higher priority should be given to consistency with the existing features. The light of a semaphore signal should be considered carefully in relation to other signals observed by a driver. The continued use of paraffin lamps or low intensity electric lights are generally not desirable, but an arrangement which leaves a mixture of high and low intensity lights is likely to be problematic to a driver at night. C4.2 Visibility requirements C4.2.1 Main signals The visibility requirements set out in section C1.3 apply to both the day and night indications of the signal. C4.2.2 Shunting and subsidiary signals The required reading distance for an independent shunting signal shall be sufficient for a driver to brake to a stand at the signal from the permissible speed applicable to the train movement. Additional requirements for a subsidiary shunt signal used for controlling movements onto an occupied line are set out in GK/RT0044. The requirements in this section are not applicable when an independent shunt signal acts as a preset shunt signal. An assessed minimum reading time for main semaphore signals is normally required in Clause B5.2. However, it is reasonable to assume that stop signals need not be assessed at the permissible speed since a driver should only need to observe semaphore stop signals after receiving a caution aspect at the distant signal. Requirements and guidance for shunting and subsidiary semaphore signals are broadly in line with clause C3.2. C4.3 Lateral positioning C4.3.1 Main and subsidiary signals The centre of the lowest element of the signal shall be as close as reasonably practicable to, but not below, driver s eye level, except as permitted below. Where essential to provide adequate visibility, it is permissible for the signal height to be increased. In deciding whether it is acceptable to use a greater height, attention shall be given to: a) the difficulty that a driver could have in relating the signal to the line to which it applies (especially where the approach to the signal is curved, with signals on parallel lines) b) the readability of the signal when the driver is close to it 42 RAIL SAFETY AND STANDARDS BOARD

45 Page 43 of 71 c) the risk of reading through to another signal ahead, instead of obeying the correct signal. It is permissible for subsidiary shunting signals to be ground mounted. RAIL SAFETY AND STANDARDS BOARD 43

46 Page 44 of 71 C4.3.2 Shunting signals Shunting signals shall usually be positioned below driver s eye level so as to be clearly differentiated from main signals. The selection of the height shall be determined by: a) limitations of structure gauge b) achieving consistency of height within a junction area c) cab sightline limitations. C4.3.3 Signs The horizontal centre of the distant board or the red target of a stop board shall usually be positioned 2.5 m (+/- 0.1 m) above rail level. Historically, the lateral positioning of main semaphore signals has been notoriously varied and therefore for consistency it could be argued that this should be perpetuated. Whilst the standard is less specific with regards to semaphore signals, the basic principles described in sections B5 and C1 still apply. Issues relating to the positioning of mechanical shunting signals are similar to those for an independent PLS, except that the use of multiple arms or discs to provide routing information can create additional difficulties in respect of clearance of structure gauge. The height above rail level quoted in clause C4.3.3 for signs acting as signals is inconsistent with the general requirement for the positioning of main signals. The figure of 2.5 m is derived from a superseded version of GK/RT0031 which stated equivalent dimensions. The inconsistency is generally justified on the basis that these signs are fitted to very simple sections of railway with relatively low speeds. The lower height permits simpler fixing in the ground. C5 Use of co-acting signals and banner repeating signals In order to maintain the alignment of signs it is recommended that they are attached to two posts so as to avoid the risk of becoming twisted. C5.1 Applications where co-acting and banner repeater signals should be considered Co-acting and banner repeating signals shall be considered for use in situations where there is an identified need for the driver to see signal aspect information but is unable to see the main signal. No guidance is provided. C5.2 Co-acting signals C5.2.1 General requirements The use of a co-acting signal shall be considered where it is impractical to position a main signal such that it is visible at long range (the start of the required reading distance) without compromising readability at close range or at the normal stopping position. Co-acting signals shall not usually be provided at a splitting distant signal unless the primary and co-acting aspects are never seen together. C5.2.2 Form of co-acting signals It is permissible to use co-acting signals with both colourlight and semaphore forms of signal. It is not permissible to use a combination of colourlight and semaphore signals to form co-acting signals. 44 RAIL SAFETY AND STANDARDS BOARD

47 Page 45 of 71 The main aspects/arms of the co-acting signals shall be the same. Any other indications required (for example route, subsidiary or RA indications) shall be positioned on either or both signals, depending on the driver s need to see the indications. The equipment performance requirement (see GK/RT0031) of the close-range signal shall be sufficient for the viewing range it is designed for. C5.2.3 Positioning of co-acting signals The longitudinal separation of the co-acting signals shall be no more than 2 m. It is permissible for co-acting signals to be positioned on opposite sides of the line to which they apply. The lateral position of the co-acting signals shall be such that there is no possibility of a driver mistakenly thinking that either applies to another line. It is permissible for a co-acting signal that is intended to be seen at long range to exceed the maximum height requirement set out elsewhere in this standard, where necessary to provide adequate visibility so long as the other signal is positioned to provide adequate visibility at short range. C5.2.4 Identification numbers GK/RT0032 sets out requirements for the unique numbering of co-acting signals. The signal head designed for close-range viewing shall usually be identified as the primary signal. The requirements for long-range and close-range viewing can sometimes create conflicts in selecting the position of a signal. Using co-acting signals may provide a solution that allows both sets of requirements to be achieved satisfactorily. There are two different circumstances that might benefit from the use of co-acting signals; either where obstructions make it impossible to achieve satisfactory long-range and close-range viewing of one signal head, or where the normal stopping point is positioned so close to the signal that the aspects are not sufficiently prominent to a driver. If a co-acting signal is required only for trains starting from rest or low speed, it may be suitable to use a type of signal designed for short-range viewing, which may be simpler and easier to install in a position best suited for the circumstances. When co-acting signals are to be used, the signal sighting committee should consider where to position any route, subsidiary or supplementary indications. The committee should determine which head the driver will be observing at the time the additional indications are to be observed. Since the signal position also acts as a stop line, there is a need to maintain a clear point beyond which a SPAD can be said to have occurred. It is therefore preferable to keep the pair of signals as close as possible longitudinally. Further guidance on lateral positioning can be found in clause C1.2. C5.3 Banner repeater signal C5.3.1 General requirements Banner repeater signals provide only limited information about the aspect of the signal ahead, which in turn restricts their usefulness as a means of improving the reading time of main signals. RAIL SAFETY AND STANDARDS BOARD 45

48 Page 46 of 71 Any proposed application of a banner repeater signal shall either: a) be supported by an assessment that identifies specific hazards to the correct observance of the signal that can be mitigated by the introduction of a banner repeater signal, or b) be to provide a performance enhancement to some or all trains. C5.3.2 Visibility requirements for banner repeater signals Where a banner repeater signal is provided, it shall be readable by the driver for a minimum of five seconds. It is permissible to determine the minimum reading distance for the banner repeater signal based on the attainable speed of those trains for which the banner repeater signal has been provided. Where a banner repeater signal is to be used in association with a signal capable of displaying a flashing aspect, the achievable reading time of the main signal shall be sufficient to ensure that the signal can be observed to be flashing. Usually the main signal shall become visible when the driver loses sight of the banner repeater. However, it is permissible to have a gap between losing sight of the banner signal and the main signal coming into view, if the effect is to significantly increase the total reading time. The gap shall be as brief as possible, subject to achieving good sighting of the banner signal, (typically no more than three seconds). C5.3.3 Lateral position of banner signal The requirements of clause C1.2 shall be applied to determine the lateral position of a banner repeater signal. There are two significant limitations of the banner repeater display; firstly the current design is classified as a category 2 device in GK/RT0031 which guarantees readability to a distance of 250 m only, secondly the ON/OFF display limits the information that can be given to the driver. Despite these limitations the banner repeater signal is the only device available that is permitted to be positioned before reaching the signal, and capable of giving some indication of the aspect displayed. As such, when used in the right way, it can serve as a valuable aid to drivers approaching a signal that has features that are less than ideal. The readability limitation has the effect of making the banner repeater signal more applicable to lower speed lines or limited to the benefit of trains travelling at a reduced speed. The display limitation has the effect of making the banner repeater signal more applicable to circumstances where there is a benefit in alerting the driver to a signal possibly displaying a stop aspect. The use of a banner repeater signal to extend the achievable reading time is not encouraged in because a train driver travelling at the permissible speed can reasonably expect to find the next signal showing either a clear aspect or a first caution. Since the banner repeater signal display cannot differentiate between these two the banner repeater signal does not provide any aspect information that is helpful. It can be argued, however, that the presence of the banner repeater signal can act to prime the driver of the approaching signal and as such could help overcome a deficiency in reading time. Recent human factors evidence suggests that no more than half a second of the required reading time can be deducted as a result of a driver being primed (this figure is not dependent on how long the banner repeater signal is visible for). The additional cost of providing a banner repeater signal to prime the driver would be assessed against a quite small benefit, unless other benefits have also been identified. Clause C5.3.1 calls for the signal sighting committee to identify the hazards that can be mitigated by the provision of a banner repeater signal. This is to ensure that specific benefits are identified to justify the cost and to avoid banner repeater signals being installed where they cannot assist the driver. 46 RAIL SAFETY AND STANDARDS BOARD

49 Page 47 of 71 The following examples represent circumstances where specific hazards may be mitigated by the introduction of a banner repeater signal. A SOY SPAD (Start On Yellow SPAD) can occur when a driver sets off from a station with a stop aspect at the next signal but concentrates on accelerating away from the station. Depending on the speed that the train has reached and the achievable reading distance, a banner repeater signal may provide additional warning of the stop aspect so as to avoid the SPAD. It is known that junctions with an approach release from yellow feature can trap a driver into thinking that the signal beyond the junction will be showing a proceed aspect. Having passed through the junction the driver may accelerate up to the permissible speed and fail to stop at the signal. As with the SOY SPAD example, a banner repeater signal may provide additional warning so as to avoid the SPAD. If a risk of the driver reading the wrong signal has been identified (perhaps because of a curved approach to a gantry) the provision of a banner repeater signal may help avoid a driver (having passed the previous signal at caution) observe the wrong signal which could be showing a proceed aspect, and start to accelerate. A signal at the exit from a tunnel or other very dark section may be difficult to read as a result of the sudden impact of daylight (see Appendix 3 for details on light adaptation). A banner repeater signal positioned within the tunnel may assist the driver if the signal is not easily observed in the bright daylight. Banner repeater signals provided for performance enhancements are normally associated with locations where an early warning of a signal having cleared from stop to proceed is considered to be worth the additional cost involved. The benefit is greatest when freight trains with relatively poor acceleration are involved. The provision of a splitting banner repeater signal may provide earlier advice of the clearance of a junction signal and give sufficient information to the driver as to the speed permitted beyond the signal. Clause C5.3.2 sets out the minimum reading time for banner repeater signals. If the banner repeater signal has been identified as benefiting only trains that have already been cautioned by previous signal aspects, then the minimum reading distance may be calculated based on the calculated reduced speed appropriate to those trains. Guidance on the lateral positioning of banner repeater signals is included under clause C1.2. C6 Platform equipment C6.1 Position and visibility of signals on platforms C6.1.1 Visibility of platform starting signals for drivers All elements of a signal designated as a platform starting signal shall usually be readable by a driver in the driving position of any train authorised to use the route, from all normal stopping positions of trains at the platform. Where this is impracticable, banner repeaters shall be provided which are readable from the normal stopping positions. Where permissible speeds allow, miniature banner repeaters meeting performance category 3 of GK/RT0031 (or, as a non-preferred option, OFF indicators) can be used. C6.1.2 Lateral position of signals on platforms Where signals on platforms are positioned below the normal height (see clauses C2.3.1 and C4.3.1), specific consideration shall be given to the risk of elements being obscured by people on the platform, as well as to other factors associated with signals positioned below the normal height. RAIL SAFETY AND STANDARDS BOARD 47

50 Page 48 of 71 It is permissible for miniature banner repeater signals to be positioned to the right of the line to which they apply if the platform is on the right-hand side and they are readable from the associated stopping position(s). No guidance is provided. C6.2 Train dispatch equipment C6.2.1 Switches and plungers Switches or plungers required for train ready to start (TRTS), right away (RA) or close doors (CD) functions on a platform shall be co-located and shall be positioned where staff carrying out train dispatch duties can adequately observe the train they are dispatching. Where platforms are bi-directional or have mid-platform signals, the layout and labelling of the switch/plunger units shall be such as to minimise the risk of operation of the wrong switch/plunger. It is permissible for duplicate switch/plunger units to be provided where more than one dispatch position is required. C6.2.2 OFF indicators OFF indicators shall be provided in situations where the guard or person in charge of the platform is unable to see the platform starter signal or banner repeater signal from all designated dispatch positions. They shall be positioned so as to be readable by the dispatch staff from the dispatch position(s). GO/RT3475 sets out requirements for assessing methods of dispatch. A banner repeater signal is not a suitable substitute for an OFF indicator if it is replaced to danger by occupation of the signal berth track circuit. Where OFF indicators are provided on bi-directional platforms or platforms equipped with mid-platform signals, the OFF indicator shall additionally indicate the signal or direction of movement to which it applies. C6.2.3 CD / RA indicators CD and RA indicators, where required, shall meet all of the following requirements: a) be co-located with any platform starting signal provided and with any banner repeater signals (see clause C6.1.1) b) be positioned such that they are readable by the driver when stationary at any normal stopping position c) be positioned such that they are readable by the person in charge of the platform. Where necessary, duplicate indicators shall be provided to achieve the requirements of this clause. The positioning of train dispatch equipment can play a critical role in reducing the risk of a SAS SPAD. When giving the signal to the driver to depart, it is essential that dispatch staff can observe the whole of the train. This may be one person (train or platform based), or a group of people located so as to observe the full length of the train. Equally important is the requirement that the platform starting signal (where provided) should be checked before the signal is given to the driver to depart. The person giving the signal (train guard or person in charge of the platform) should be able to see the signal directly or some indication ( OFF indicator or banner repeater signal) at the time that the signal is given. It is not necessary for other dispatch staff to see the signal indication but their duties should clearly 48 RAIL SAFETY AND STANDARDS BOARD

51 Page 49 of 71 state that they are communicating only with the person in charge and not directly with the driver. It is likely that the dispatch assessment process mandated in GO/RT3475 provides input into the need for train dispatch equipment needed. C6.3 DOO monitors and mirrors Monitors and mirrors for DOO shall be positioned: a) so as not to obscure the view of any signal for drivers of stopping or nonstopping trains b) so as to minimise the risk of a SAS SPAD. Technical requirements for DOO monitors and mirrors are set out in GE/RT8060. There are several issues to consider when positioning monitors and mirrors for DOO operation. These issues may lead to conflicts in optimum positioning. The following guidance may assist with the final selection. The requirement to avoid obscuring the line of sight to signals by drivers of both stopping and non-stopping trains should be the first priority. The requirement to minimise the risk of a SAS SPAD calls for a consideration of the possibility of a driver overlooking the need to observe the signal, having closed the train doors. This risk is believed to be greatest when the driver is required to look in a direction significantly different than that of the signal. It therefore follows that monitors and mirrors positioned so that the driver looks forward (forward view) are preferable. Unfortunately, mirrors, and particularly monitors, should ideally be positioned as close to the driver as possible, which can best be achieved when positioned alongside the cab (side view). In this position the image will appear larger. It has also been noted that problems of glare from the sun are sometimes avoided by the presence of the train alongside the monitors. Mirrors and monitors positioned under a canopy or other permanent form of shade should be better protected from sunlight problems. If the train stop position is set well back from the signal (25 m or more) then there may be time for a driver starting off having failed to observe the signal, to still have time to observe the signal and attempt to stop. It therefore follows that the arrangement with the highest risk is side-view monitors positioned close to the signal. Options for repeating signal aspect information alongside the monitor or mirror housing could be considered. Currently the only method used is to place a miniature banner repeater unit under the housing. Other options such as a miniature main signal may become available. Right-hand platforms introduce additional issues if monitors are to be used. Positioning the monitors on the right-hand side results in a greater distance from the driver to the monitors; positioning on the left-hand side can increase the risk of the wrong train doors released. Neither option is ideal, but it may be possible to use larger monitors to compensate for the extra distance. A consistent approach to the positioning of monitors at right-hand platforms is advisable. Finally, in an ideal world, it is desirable for all monitor or mirror units on a particular DOO scheme or line of route to be all forward view or all side view so as to avoid confusion over the required train stopping position relative to the unit. This may not be possible and therefore other stop markers may help clarify the stop position at each site. C6.4 Train stopping positions No car stop marker or DOO monitor unit shall be positioned such that a train is required to stop within 25 m of the platform starting signal, except as permitted below. RAIL SAFETY AND STANDARDS BOARD 49

52 Page 50 of 71 It is permissible for the normal stopping position of a train to be within 25 m of the platform starting signal where the signal is sufficiently conspicuous such that a driver in the driving position is aware of the aspect displayed while looking at the route ahead. Where there is doubt as to whether this is achievable, consideration shall be given to the use of additional control measures to reduce the likelihood or consequence of a SPAD. Such measures include, but are not limited to: a) use of RA indicators as part of the train dispatch process b) a train protection system to stop trains short of any area of conflict (as set out in GI/RT7006) c) a suitably positioned co-acting signal. The purpose of the requirements in this clause is to reduce the risk of SAS SPADs and SOY SPADs. In both cases, the concern is that the additional duties and distractions involved in a station stop result in a driver setting off without observing the platform starting signal. If it was at red a SAS SPAD occurs, if it was at single yellow the next signal is likely to be SPADed (SOY SPAD). It is important to ensure that platform starting signals are sufficiently prominent in the driver s eye that the driver is unlikely to set off without observing the signal. Appendix 1 identifies features of human vision and perception that suggest that such signals should be within an 8 o cone of the driver s line of sight to achieve prominence. The table below identifies the radius of an 8 o cone at a range of distances from the driver s eye. Longitudinal distance of signal from driver s eye Lateral distance of 8 viewing angle from line of sight 5 m 0.70 m 6 m 0.84 m 7 m 0.98 m 8 m 1.12 m 9 m 1.26 m 10 m 1.41 m 11 m 1.55 m 12 m 1.69 m 13 m 1.83 m 14 m 1.97 m 15 m 2.11 m 16 m 2.25 m 17 m 2.39 m 18 m 2.53 m 19 m 2.67 m 20 m 2.81 m 21 m 2.95 m 22 m 3.09 m 23 m 3.23 m 24 m 3.37 m 25 m 3.51 m Typical signal positions Red at 3.3 m above rail level and 2.1 m from left-hand rail; within 8 at m in front of driver. Red at 5.1 m above rail level and 0.9 m from left-hand rail; within 8 at m in front of driver. Red at 3.3 m above rail level and 2.1 m from right-hand rail; within 8 at m in front of driver. 50 RAIL SAFETY AND STANDARDS BOARD

53 Page 51 of 71 Table 1 Conversion of viewing angle into lateral distance It can be seen from Table 1 that, depending on the lateral position of the signal, the normal stopping point may need to be as far as 25 m back from the signal to ensure that the signal is sufficiently prominent. The above dimensions assume that the driver is looking straight ahead. For non- DOO trains this is broadly reasonable. However, when the driver is required to observe monitors or mirrors, the line of sight at the moment that power is taken could be dependent on the location of the monitors/mirrors. In this case it may be appropriate to consider providing signal aspect information alongside the monitors or mirrors. This could be achieved with a co-acting signal arrangement (possibly using a miniature signal) or by positioning a miniature banner repeater alongside the mirror/monitor. At New Cross a miniature banner repeater is in use, and it only displays an aspect when a train is at the platform because the repeater is aligned at right angles to the platform edge and there is an adjacent track where trains could stop. At non-doo platforms, an RA indicator is a powerful tool for reducing the SPAD risk. This is achieved by combining the station duties complete signal with the signal proceed aspect. Only when both conditions are present does the RA indication illuminate. C7 Miscellaneous technical requirements C7.1 Colour of structures and fittings The colours usually applied to signal structures and fittings are set out in GK/RT0031. It is permissible to paint signal structures in a distinctive colour to aid drivers in locating a signal, and the line to which it applies, in a complex area. Permitted arrangements are: a) black for the horizontal part of a gantry or cantilever, where this will distinguish it from other structures that are coloured grey (for example, OLE) b) white and black hoops for a post. Overuse of such a measure can render it ineffective. There is increasing evidence to indicate that the outline of the signal structure is an aid to identifying signals. Depending on the background that the structure will be seen against, painting the horizontal section of a gantry or cantilever structure black may be considered helpful in establishing the outline of the structure. The use of black and white hoops is less commonly used but has been applied in some parts of the country in the past. C7.2 Countdown markers It is permissible to provide countdown markers, as set out in GI/RT7033, on the approach to signals where this is felt to be beneficial. Such markers shall be provided as a sequence of three signs, positioned nominally 100 m apart with the last one nominally 100 m before the signal. Other arrangements are permissible where it is considered that this will be of greater benefit to the driver. The use of countdown markers shall usually be determined as part of the risk assessment process required by GI/RT7006. Overuse of countdown markers can render them ineffective. Whilst recognising that in some countries, countdown markers are used on the approach to all main signals, it is recommended that these markers are used only where difficulties have been (or are anticipated to be) experienced. In this way RAIL SAFETY AND STANDARDS BOARD 51

54 Page 52 of 71 their existence should help prompt the driver to the fact that the approach to the signal is more problematic than usual. The positioning dimensions quoted should be consistent nationally to help avoid misleading the driver on the approach to the signal. If greater warning distance is thought to be required it may be appropriate to extend the sequence with additional boards at 400 m, 500 m, etc rather than increasing the distance between boards. C7.3 Signal reminder signs It is permissible to provide a signal reminder sign, as set out in GI/RT7033, on the approach to signals where there is a significant risk of a driver forgetting that he is approaching a signal at danger (for example, due to distractions after passing the previous signal at caution, such as a station stop). The position of such signs shall be selected on a site-specific basis such that the driver, having been reminded, is able to take action to stop at the signal. The use of signal reminder signs shall usually be determined as part of the risk assessment process required by GI/RT7006. Overuse of signal reminder signs can render them ineffective. Signal reminder signs should be recognised as having limited benefit. Signs in general are not as prominent as a lit signal and as such are not ideal as a means of attracting the attention of a driver. Their use may be justified if it is felt that the driver may become distracted by duties other than obeying signals. In this way a driver (for example, having dealt with a station stop) may benefit from such a sign as a prompt to consider the aspect of the previous signal and hence determine how to control the speed of the train. Positioning of new signals should take such risks into consideration and hence ensure that signal reminder signs are not needed. C7.4 Sighting boards, hoods and blinds It is permissible for sighting boards, extended hoods or vertical blinds to be used where it will improve the readability of signals under some or all ambient lighting conditions. The effect shall be assessed to avoid any reduction in readability from any driving position of the driver of a train approaching the signal. Consideration shall be given to the need to ensure that such measures do not degrade the readability of other signals in the area. Consideration shall be given to masking a source of interference where this is more readily achieved. It is recommended that gaps between main signal heads and other elements are filled in as part of the backboard. This will help to present the various elements as one signal and reduce the risk of light being seen through a small gap appearing as if it were on aspect. Backboards on main colourlight signals provide vital contrast between the lit aspect and the immediate area around as seen by the driver of an approaching train (see also Appendix 3). Increasing the size of the backboard, or providing a separate larger backboard behind the signal, can significantly improve the level of contrast, particularly as the viewing distance increases. The use of white or blue/white edge to the backboard has been shown to be of benefit only at close-range viewing. At long-range viewing such a colour scheme 52 RAIL SAFETY AND STANDARDS BOARD

55 Page 53 of 71 may actually reduce the readability of the signal. Care should be taken therefore to assess the specific requirements before introducing such features. If improved contrast is required for a semaphore signal, it is usual practice to arrange for a white background to be provided. The length of the hood on main colourlight signals is a compromise between giving maximum protection from the effects of sunlight on the lens and the risk of the hood forming an obstruction to viewing aspects, particularly at close range. In general, hoods should be as long as possible, consistent with avoiding closerange viewing problems. Blinds have been used where specific problems relating to drivers observing the wrong signal (or other signal-like light) have been identified. Blinds usually consist of a fixed board positioned so as to screen the unwanted signal from the view of the driver. C7.5 Line identifiers C7.5.1 Form and use of line identifiers The use of line identifiers, as set out in GI/RT7033, is permitted on parallel lines where this will reduce the risk of confusion as to which signal applies to which line. Where line identifiers are used they shall be applied to all parallel lines, numbered or lettered in a fashion that is logical to drivers, and positioned in a consistent manner. They shall be positioned so as to be visible to drivers as follows (one, or more than one, of the arrangements can be applied): a) above the signals b) on the rear of gantries carrying signals for opposite direction movements, or on other structures c) between the running rails, in the area known as the four foot d) on posts. The use of line identifiers shall usually be determined as part of the risk assessment process set out in GI/RT7006. C7.5.2 Application of line identifiers when alphanumeric route indicators are also in use Where alphanumeric route indicators are also in use, the characters for the line identifiers shall be the same as those displayed by the route indicators (that is, the route indicators shall indicate the line identifier character for the line on which the next signal is to be found). Line identifiers have been introduced on several sections of complex railways. The benefit provided by these is generally considered to be as an aid in associating signals with the correct line. Their use, therefore, is likely to be restricted to sections of railway with many parallel lines, typically the approaches to large terminal stations. The precise positioning of the signs relative to signals is likely to require careful consideration and consultation. Consideration should also be given to the readability of the signs at night. See also Appendix 3. C7.6 Arrow signs The use of an arrow sign fitted close to the signal head and pointing to the left or right to indicate to which track the signal applies, is permitted. Its use shall be RAIL SAFETY AND STANDARDS BOARD 53

56 Page 54 of 71 limited to signals where a risk of confusion about which track it applies to has been identified and no other measures to reduce the risk are practicable. Arrows attached to right-hand mounted PLSs has been standard practice for some years. In a few places arrows have now been fitted retrospectively to righthand mounted main signals that have caused problems. Clause C1.2.2 should normally prevent new signals being positioned on the righthand side if there is a likelihood of confusion. The application of arrow signs should therefore be restricted to existing signals or new signals with a compelling reason for being positioned to the right. C7.7 SPAD indicators The SPAD indicator shall be aligned and positioned so as to: a) maximise the likelihood of an unauthorised movement being brought to a stand before reaching any point of conflict b) bring authorised movements to a stand, where indicators are provided at more than one conflicting signal, in the event of a SPAD being detected at another signal. The use of a co-acting indicator is permitted if conflicts arise in meeting these requirements. The requirements of clauses B5.2, B5.3, B5.4 and C1.3 shall not apply to SPAD indicators. Overrun risk assessments (see GI/RT7006) may occasionally identify the need for a SPAD indicator. When positioning a SPAD indicator it may be helpful to consider the limited circumstances in which it can provide assistance. Given that the indicator will only illuminate, once the train is proved to have passed the main signal, the concept of minimum reading time is not appropriate. C7.8 Point indicators and crossing indicators Visibility requirements shall be as set out in clause C1.3 so far as they are applicable. The lateral position shall be as set out in clause C2.3. A train approaching a point indicator or a crossing indicator should be travelling at a speed such that it can stop, if necessary. Therefore, it is reasonable to provide a reading distance similar to a PLS. C7.9 Anti-vandalism measures The readability of a signal shall not be affected by any anti-vandalism measures applied to the signal. It is inevitable that when providing protection to a signal from acts of vandalism, the readability is affected. It is important to ensure that the readability remains acceptable. The use of polycarbonate lenses have a number of side effects that should be considered. Firstly, a new polycarbonate lens provides less intensity when seen off axis, which may be significant if observed on a tight bend or when close up. 54 RAIL SAFETY AND STANDARDS BOARD

57 Page 55 of 71 Cleaning the lens can cause scratching which will disperse light. The polycarbonate also becomes cloudy in sunlight over a period of a few years, so reducing the light output. C7.10 Use of new designs of signs GI/RT7033 sets out requirements associated with proposals to introduce new designs for signs. GI/RT7033 has been designed to control the way in which new lineside signs are introduced. Whilst recognising the need for new signs to be created, the standard attempts to introduce them in a nationally consistent manner. C7.11 Positioning signalling equipment not identified in GK/RT0031 Where signalling equipment is to be provided that has not been identified in GK/RT0031, it shall be: a) positioned in accordance with clause B5.1 b) approved for the intended use by the infrastructure controller c) positioned in accordance with any requirements specified by the infrastructure controller. There will be occasions when a signal sighting committee is called upon to position equipment that is not identified in Railway Group Standards as suitable for use as an item of signal display equipment. The most likely reason is that the item is either experimental or has only been approved since the latest issue of the relevant standard. In such circumstances there will be no specific requirements in Railway Group Standards to control the positioning of the equipment. Clause C7.11 is designed to provide sufficient controls to permit new or experimental equipment to be introduced in an acceptable manner. RAIL SAFETY AND STANDARDS BOARD 55

58 Page 56 of 71 Appendix 1 Required reading distance summary of issues The process of assessing what is the minimum requirement for reading distance, what is achievable on site and what can be done to improve the reading time, is one of the most demanding parts of the duties carried out by the signal sighting committee. The flow chart and tables in this appendix are designed to summarise the requirements set out in and to provide additional guidance on making the most appropriate selection of mitigating measures. The flow chart summarises the key steps which should be taken, in order to come up with a reading distance considered acceptable and recorded as the required reading distance. Within the flow chart there are references to mitigating factors that should be considered to improve the reading time/distance. These measures are listed in Tables 1 and 2, together with some possible consequences (positive and negative) that may result from the introduction of the measure. Determine the minimum reading distance (MRD) [B5.2, B5.3, B5.4] Determine the achievable reading distance (ARD) [B5.5] Is ARD MRD [C1.3.1] YES NO Consider suitable mitigating measures [table 1] Is ARD MRD [C1.3.1] YES NO Consider departure from requirements of C1.3.1 [table 2] Record ARD as Required Reading Distance [C1.3.1] END Chart 1 Determining the Required Reading Distance 56 RAIL SAFETY AND STANDARDS BOARD

59 Page 57 of 71 Measure Positive factors Negative factors Relocate signal laterally Relocate signal longitudinally Separate MRT for each aspect Use of attainable speed Use of co-actor signals Use of banner repeater signal Reduction in permissible speed Apply special signalling controls Reduction in turn-out speed at junction Relatively simple solution for new signals. Can give significant improvement on curved track sections. More accurately reflects the reading distance requirements. Avoiding the introduction of other measures should avoid the introduction of their negative factors. More accurately reflects the reading distance requirements. Avoiding the introduction of other measures should avoid the introduction of their negative factors. Can be effective solution to a specific obstruction. Allows the primary signal head to be optimised for close-range viewing. Rules for positioning banner repeater quite flexible. Provides a priming function to approaching drivers. Potentially can solve any problem of inadequate reading time. Can selectively reduce train speed to increase reading time without affecting all trains. Can help to simplify junction signalling requirements or avoid advanced warning. Limited occasions when this can help. A nonstandard position may introduce confusion as to which line it refers to. Disturbing a regular signal spacing can create a trap to drivers. Assessment process may be more complex. Likely to result in the ARD being only just greater than the MRD. Likely to result in the ARD being only just greater than the MRD. Future train designs may invalidate the acceleration calculations made. Limited occasions when this can help. Potential to cause confusion if parallel lines signalled in the same direction. Limitations of display information and viewing range significantly restrict the circumstances where it will be beneficial. Likely to increase the number of speed changes on the route. Reduction in capacity may increase the number of stop aspects observed by drivers. Some reduction in capacity may occur. Possible risk of anticipation if drivers see aspects step up regularly. Some reduction in capacity may occur. Table 1 Mitigation measures RAIL SAFETY AND STANDARDS BOARD 57

60 Page 58 of 71 Measure Positive factors Negative factors Modify junction signalling controls Redesign infrastructure Reduction in signalling facilities Can help to simplify junction signalling requirements or avoid advanced warning. A revised layout may simplify signalling requirements. Removing reversible signalling, for example, may reduce risks of misreading and permit a reduced MRT. Some reduction in capacity may occur. Likely to result in less than optimal layout. Likely to result in less than optimal layout. Measure Positive factors Negative factors Departure from part C of to permit ARD < MRD Avoiding the introduction of other measures may avoid introducing negative factors associated with those measures that are felt to be unacceptable. Only suitable where careful consideration of the risks associated with the signal suggest that it is more effective than the application of other measures. Table 2 Departure from the requirements of clause C RAIL SAFETY AND STANDARDS BOARD

61 Page 59 of 71 Appendix 2 Typical forms for recording signal sighting details 2.1 Introduction This appendix identifies a recommended collection of forms that can be used to form part of the signal sighting records for a signal. does not mandate how the forms appear and the details here are intended to outline how and when the forms should be used. Although some signals will justify the use of all the forms listed below, it is not intended that all the forms will be needed for every signal. 2.2 Signal Sighting Form The signal sighting form is intended to record all the physical design features as agreed by the committee. The form should include a drawing showing the position of all the signal elements, the type of structure and dimensions to the running rails. It is recommended that all signage that is to be fixed to the structure be shown so as to clearly identify their positions relative to the signal. Space should be provided for signatures of all the committee members, the checker and the person authorising the plan for the infrastructure controller. 2.3 Minimum Reading Time (MRT) Assessment Form The MRT form is intended to record details of any risks identified that may require additional reading time to be applied to the signal as set out in clause B5.2 of the standard. The form should provide evidence that risks have been considered as not appropriate, removed, or mitigated (by adding time or by modification). The form should clearly identify the assessed minimum reading time needed. 2.4 Obscuration diagram The obscuration diagram is intended to record any interruptions to the view of the signal during the required reading distance. Having recorded the interruptions, approved criteria for assessing the acceptability of the signal can then be determined and recorded on the form. 2.5 Departures from Part C This form can be used to formally record any details where departures from Part C of are to be made. The form should be used to identify the specific clause and issues involved, and can be used to capture the justification. Space for approval by the infrastructure controller should also be provided. 2.6 Calculations and Assumptions Form This form can be used to record any calculations or assumptions made as part of the signal sighting process so that they can be recorded for future use. Calculations may include such things as attainable speed to be used. Assumptions may include statements about types of rolling stock using the route or service patterns anticipated. 2.7 Numbering of forms Each form should be uniquely numbered to aid future reference. One method is to keep the same reference number for a particular signal and to add an identifying suffix for each form (/1 for the sighting form; /2 for the MRT form; /3 for the obscuration diagram; /4 for a departure form; /5 for any calculation and assumptions form). RAIL SAFETY AND STANDARDS BOARD 59

62 Page 60 of 71 Appendix 3 Human Vision and Perception 3.1 Introduction Aim A signal sighting committee s role is to verify the proposed location and form of all new, modified and re-positioned signals, indicators and signs. This appendix aims to provide signal sighting committee members and signal sighting engineers with an understanding of the capabilities and limitations of the human visual system that may be relevant when making signal sighting decisions or investigating causes of SPADs Vision and perception Throughout this appendix, vision refers to the actual seeing of raw visual information, whereas perception refers to the recognition of, or making sense of, that visual information. Vision is a bottom-up, or data-driven process determined by the physiological function of the eyes and optic nerve. Perception is a top-down, or conceptually-driven process determined by cognitive function and influenced by knowledge and expectation. Visual illusions like the Necker cube (Figure 1) demonstrates the distinction between vision and perception. In this example, the raw data of the cube (that is, the lines that make up the cube) are equally visible to the people looking at it, but the perception of the object is ambiguous. For example, people may have different interpretations regarding which way the cube is facing. Figure 1 The Necker cube Cubes can be drawn with solid walls, of course, which then resolves the ambiguity (Figure 2). Figure 2 Necker cube with solid walls A human performance model of signal reading Knowledge of the driving task and the information processing involved has been combined to develop a model of signal reading. While this description of the signal reading task may not be universal and unvarying, it can be considered an appropriate engineering approximation of the behavioural stages involved in the task. The model is illustrated in Figure RAIL SAFETY AND STANDARDS BOARD

63 Page 61 of 71 INPUT COGNITION OUTPUT Vigilance Detection Recognition Interpretation Signal sighting is a vigilance task that requires sustained levels of attention and alertness Visible features of a signal are detected in the environment 1. Signal perception the signal s form is identified and discriminated from surrounding objects 2. Association with line signal is recognised as appropriate to the driver s route Signal aspect is read and an appropriate response is chosen TIME Action Driver responds to the signal Figure 3 Illustration of driver information processing during the signalreading task This signal reading model makes the distinction between vision (detection) and perception (recognition). At the signal detection stage, the raw data of a signal are visible to the driver. At the signal recognition stage, these data are perceived, that is, the signal s form is identified and discriminated from its background and the signal is recognised as appropriate to a particular line. Consistent with the different meanings of vision and perception, objects can vary in their visibility and their perceptibility. In terms of signal sighting, factors that affect a signal s visibility are those that influence the detection of the signal, that is, getting an image onto the observer s retina. Whether or not that image is then recognised as a signal and read accurately is determined by the perceptibility of the signal. Both visibility and perceptibility are properties of the signal, its design, and its context Structure of appendix The stages of the information-processing model of signal reading that relate to vision and perception have been used to structure the remainder of this appendix. Section 2 concerns the visibility of signals and describes how the processes of vision may influence the visibility of a signal to the observer. Section 3 discusses factors that affect the perceptibility of signals and describes how perceptual processes may influence the ease with which an observer is able to accurately associate a signal with the line. Some issues are relevant to signal design, some to SPAD investigation, and some apply to both activities. Codes have been provided in the text that denote the activity to which each topic is applicable: (D) (I) Design Investigation (D I) Design and investigation RAIL SAFETY AND STANDARDS BOARD 61

64 Page 62 of The visibility of signals Overview The effectiveness of an observer s visual system in detecting the existence of a target will depend upon the object s position in the observer s visual field, its contrast with its background, its luminance properties, and the observer s adaptation to the illumination level of the environment. It is also influenced by the processes relating to colour vision, visual accommodation, and visual acuity. Each of these issues is described below Field of vision The field of vision, or visual field, is the area of the visual environment that is registered by the eyes when both eyes and head are held still. The normal extent of the visual field is approximately 135 degrees in the vertical plane and 200 degrees in the horizontal plane. The visual field is normally divided into central and peripheral regions: the central field being the area that provides detailed information. This extends from the central point (0 degrees) to approximately 30 degrees at each eye. The peripheral field extends from 30 degrees out to the edge of the visual field. Objects are seen more quickly and identified more accurately if they are positioned towards the centre of the observer s field of vision, as this is where our sensitivity to contrast is highest. Peripheral vision is particularly sensitive to movement and light. L R Figure 4 Field of view In the diagram above, the two shaded regions represent the view from the left eye (L) and the right eye (R) respectively. The darker shaded region represents the region of binocular overlap. The oval in the centre represents the central field of vision. Research has shown that vehicle drivers search for signs/signals towards the centre of the field of vision. As approach speed increases, drivers demonstrate a tunnel vision effect and focus only on objects in a field of + 8 o from the direction of travel Relevance Drivers become increasingly dependent on central vision for signal detection at increasing train speeds, and even minor distractions can reduce the visibility of the signal if it is viewed towards the peripheral field of vision. (D I) Because of our sensitivity to movement in the peripheral field, the presence of clutter to the sides of the running line, for example, fence posts, lamp-posts, traffic, or non-signal lights, such as house, factory or security lights, can be highly distracting. (D I) 62 RAIL SAFETY AND STANDARDS BOARD

65 Page 63 of Implications Signals should be at a height and distance from the running line that permits them to be viewed towards the centre of the field of vision. (D) + 8 o In practical terms, this equates to positioning signals within + 8 o of the line of travel at the sighting distance and at the drivers eye level. (D) Figure 5 Signal positioning Car stop signs should be positioned such that, if practicable, platform starting signals and OFF indicators can be seen in the driver s central field of vision. (D) If possible, clutter and non-signal lights in a driver s field of view should be screened off or removed so that they do not cause distraction. (D I) Contrast sensitivity We see objects not just because of their absolute brightness, but also by their contrast with the surrounding environment. When contrast is high, objects are more conspicuous. The contrast between an object and its background is especially important when we need to detect it from a distance. Figure 6 Contrast sensitivity (The white rectangle on the left is more conspicuous than the grey rectangle on the right because it is more highly contrasted with its background.) Object orientation also affects our sensitivity to contrast. Targets that are normally presented (that is, at right angles) create greater contrast between themselves and their background than targets presented at oblique angles Relevance During the day, the contrast between a dark or cluttered background and a signal may be low, making signals less visible. (D I) RAIL SAFETY AND STANDARDS BOARD 63

66 Page 64 of 71 Signal is located here Figure 7 Low contrast due to background clutter makes signal hard to see Implications The use of matt black sighting boards, or backplates, can enhance the contrast between signal aspects and light, or cluttered backgrounds. Research has shown that larger backplates (1.5 times the current standard size of a 4-aspect signal backplate) can also improve the visibility of signal aspects, due to the increased contrast between the signal and its background. (D) White, or blue and white, backplate borders have been used in the past to try to draw attention to problem signals. However, unless the approach speed is slow (15 mph or under) and the view uncluttered, borders have the opposite effect as they merely serve to reduce the apparent size of the black backplate, thereby reducing contrast and visibility. (D I) This is because of the way the visual system processes light/dark boundaries. At their current size, borders blur into their background unless they are viewed close up. In order to be seen from a distance useful to train drivers, borders would need to be impractically large (approx. 500mm wide) (See 3.2.8). (D) However, if the signal background is dark (such as a bridge), painting the area behind the black signal backplate white will draw the driver s eye to the signal and make it more conspicuous, as long as the white area is much larger than the signal. An example of where this has been successful is at signal SY289 at Water Orton. (D) Figure 8 Signal SY289 at Water Orton. The white painted background enhances signal conspicuity. 64 RAIL SAFETY AND STANDARDS BOARD

67 Page 65 of 71 Signals should be oriented so as to be normal to the direction of approach for as much of the approach time as possible. This enhances the contrast between a signal and its background. (D) Luminance sensitivity The brighter a light source, the further the distance from which it can be seen. Brightness refers to the luminous intensity of the light emitted from a target. Research has shown that observers are more sensitive to spots of light when background illumination is low (for example, night time darkness) Relevance The brightness, or luminous intensity, of all signal aspects is fixed during the manufacturing process. The luminous intensity for signal aspects specified in the Railway Group Standard is designed for daylight conditions. However, because signals appear brighter against a dark background, they may dazzle drivers in night-time conditions. This effect is exacerbated by dark adaptation (see 3.2.5). This is thought to be a particular problem with the new LED signals, the luminance intensity of which is greater than traditional filament bulbs. (D I) Implication It may be appropriate to consider running LED signals at reduced power at night. (D) Light adaptation Sensitivity to light depends on the eye s state of adaptation. The visual system takes time to adjust after a change in illumination level. The eyes are least sensitive to spots of light (for example, signal aspects) immediately after the change. We adapt to scotopic (monochromatic) stimuli rapidly, within 200 msec. However, it takes several seconds for the eyes to accurately see objects in colour, reaching a maximum sensitivity after around 3 minutes, and levelling off after 10 minutes Relevance On emerging from a tunnel, the increase in illumination on a bright day may render signals that are positioned just beyond the end of the tunnel difficult, or even impossible, to see or read accurately. (D I) Figure 9 Signals just beyond tunnel exits may be difficult to see in bright sunlight Implication Positioning signals at tunnel exits should be avoided. The degree of light adaptation depends on tunnel length, whether or not there is lighting in the tunnel, and environmental conditions (brightness and direction of sunlight); but as a general rule, if a train will be in a dark tunnel for 5 minutes or more, signals should be positioned at least 5 seconds beyond tunnel exits to allow a degree of adaptation to occur. For example, for a line speed of 60 mph, this equates to a distance of just over 134 m. (D) Colour vision Colour vision helps an observer to distinguish between an object and its background and makes it easier to discriminate object details. RAIL SAFETY AND STANDARDS BOARD 65

68 Page 66 of 71 There are two types of visual receptor cells, or photoreceptors, in the retina: cones and rods. The cones are specialised for colour vision and for sharpness of vision. There are three types of cone in the eye, which are sensitive to each of the three primary colours: red, green and blue. Many stimuli activate two or three cone types, enabling us to see other colours. We are differentially sensitive to colours of different wavelengths. The human eye is most sensitive to yellow and green light, and least sensitive to blue and red light. Human sensitivity to light of equal intensity Wavelength (nm) Figure 10 Human colour sensitivity The colour of an object varies depending on the conditions under which it is viewed. Coloured objects or lights can be perceived as having changed colour with changes in illumination level. At night, because of the distribution of colour receptors in the retina, small or distant objects can occasionally lose their colour. This is a particular problem with yellow lights, which might be confused for white lights and vice versa. Colour blindness is the inability to differentiate between certain colours. This is an inherited disorder and affects men more commonly than women. The most common type is red-green colour blindness. This occurs in 8 percent of males and 0.4 percent of females. It occurs when either the red or green cone cells are not present or not functioning properly. People with this disorder are not completely unable to see red or green, but often confuse the two colours Relevance Red signal aspects may be less noticeable to drivers than green or yellow signal aspects. This may give rise to read-across or read-through errors to less restrictive aspects, as drivers may differentially pick up the most visible signal. (I) Signal aspects may appear to be a different colour in bright sunlight. At night, there may be a risk of misreading a white non-signal light as a yellow aspect and vice-versa. (I) Implications SPAD investigations should be aware of the possibility of read-across or readthrough to yellow and green aspects. (I) Signals should be fitted with hoods, where appropriate, as specified in clause C7.4 of. (D) Signal-like lights in the driver s field of view should be screened off, wherever possible. (D) 66 RAIL SAFETY AND STANDARDS BOARD

69 Page 67 of Afterimages Afterimages are seen under a variety of conditions when one visual stimulus field is followed by a second visual stimulus field. If you stare at a shape of a given colour for several seconds, then shift your gaze away you will see a negative afterimage. For example, stare at the red dot on the left for about 8 seconds, then blink and look elsewhere at this page. A green afterimage will appear. Under normal conditions, afterimages do not intrude on visual processing. However, an observer exposed to a relatively bright source of light, especially at night, may experience afterimages that may be confused with the actual signal Relevance Drivers occasionally report seeing red signals change to green when they have been staring at them for a while. It is plausible that what they might actually be seeing is an afterimage. (I) Implications SPAD investigations should not rule out the possibility that a driver s reading of a signal aspect may have been affected by afterimages. (I) Visual acuity Visual acuity (or spatial resolution) is our ability to discriminate very fine or small detail, and is the responsibility of a part of the retina called the fovea. Normal visual acuity (20/20 vision) is usually defined as the ability to resolve a spatial pattern separated by a visual angle of one minute of arc. Since one degree contains sixty minutes, a visual angle of one minute of arc is 1/60 of a degree. The spatial resolution limit is derived from the fact that one degree of a scene is projected across 288 micrometers of the retina by the eye's lens. In these 288 micrometers, there are 120 colour sensing cone cells. Therefore, if more than 120 alternating white and black lines are crowded side-by-side in a single degree of viewing space, they appear as a single grey blob to the human eye. A person with normal visual acuity (that is, 20/20 vision) is just able to decipher a letter that subtends a visual angle of 5 minutes of arc (written 5') at the eye (5' of arc is 5/60 of a degree). It does not matter how far away something is from the eye; if it subtends an angle of 5' of arc at the eye, then a person with 20/20 visual acuity should just be able to determine what it is. However, this is only a threshold value. Reliable letter discrimination requires a much bigger angle, for example 20, and certainly no smaller than 16. Visual acuity is superior for objects that are presented in the central field of vision, and for objects that are highly illuminated. Visual acuity decreases as velocity between object and observer increases. However, the ability to resolve fine detail of moving targets improves rapidly with practice Relevance Drivers should be able to distinguish detailed features of signals and signs more effectively in daylight conditions than in night-time conditions, and more effectively at lower speed, than at higher speeds. (I) Lineside signage that contains lettering or numbering is only useful to the driver if the characters can be discriminated from a distance. (D) RAIL SAFETY AND STANDARDS BOARD 67

70 Page 68 of Implications Signals should be located in the driver s central field of vision. (D) Any numbers or text on lineside signage should be large enough to be read from a distance on approach (that is, characters should subtend a visual angle of around 20 minutes of arc). In practical terms, at a linespeed of 60 mph, in order that a driver can read text on a sign 4 seconds in rear of that sign, character height should be at least 624 mm. See following calculation of character height: (D) 1 mile = m 60 mph = m per hr 60 mph = m per second 4 seconds viewing time = m The letter s visual angle subtended at the eye is 20 of arc (one-half of which is 10 ) d is the distance along the line of sight from the driver s eye to lineside sign = m h is one-half the height of the character in mm Since the line of sight and the plane of the lineside sign form a right angle, simple trigonometry can be used: 1. tan 10 = h/d 2. h = d * tan 10 = 312 mm 3. 2h = total height of a character at m = 624 mm Visual accommodation The eye can only focus at one distance at any given moment. Objects at other distances will be blurred; with the degree of blur increasing the further objects are from the point of focus. Focus is accomplished in the eye by changing the curvature and centre thickness of the lens, a process called accommodation. To focus on a distant object, the lens flattens, causing the refractive power of the eye to decrease. To focus on a nearby object, the curvature of the lens increases, raising total refractive power and maintaining image sharpness. The adjustment of the eye to focus on an object at one distance, and subsequently on another at a different distance, takes time. With age, our ability to accommodate near and far objects deteriorates. For example, research has shown that, on average, a 28-year-old can adjust accommodation from infinity to a fixed 25 cm viewing distance in 0.8 seconds, whereas a 41-year-old takes around 2 seconds to complete the same task. It is likely that accommodating continuous and rapidly changing viewing distances (such as those encountered in train driving) takes longer than this, but information on this is still very limited. Errors in accommodation are least likely when an object is well illuminated, for example, in daylight Relevance Older drivers may have difficulty in accommodating signals, particularly at high train speeds. (I) Implications Signals should be positioned so that there is sufficient time for the driver to focus on the signal. Minimum reading time needs to account for speed of approach. (D) Consideration should be given to the age of drivers and how deteriorating vision can affect the ability of older drivers to focus on signals, especially at faster speeds. (I) 68 RAIL SAFETY AND STANDARDS BOARD

71 Page 69 of Perceptibility of signals Overview An observer s ability to interpret visual stimuli is influenced by knowledge and expectation. The interaction between top-down and bottom-up processing is such that object size and contrast will facilitate its perception as a foreground object, and a variety of cues from the environment are used to enable judgement of distance, form, visual patterns, and position. This section examines the perceptual processes that may influence a driver s ability both to identify a signal, and to accurately associate a signal with the appropriate route. This is of particular importance when making signal sighting decisions about parallel signals Distance perception and size constancy We are able to judge the absolute distance and relative distance of objects. Absolute distance refers to the distance away from the observer that an object is located how far away a particular signal is from the train, for example. Relative distance refers to the distance between two objects the distance between a line and a signal, for example. To determine an object s distance from us or from another object, we use cues from the visual system. Monocular cues are those that only require the use of one eye. Binocular cues require the two eyes to be used together. Oculomotor cues depend on the sensation of muscular contraction of the muscles around the eye, as used in accommodation. The relative size of retinal images can be thought of as a cue for distance perception. However, if you watch a person walk down a road, he or she does not appear to shrink to half size each time their distance from you doubles. The traditional view of why this is so is that the brain takes account of the perceived distance of an object and scales up perceptual size accordingly. An alternative view is that we judge the size of objects by the scale of their backgrounds. This is demonstrated by the picture on the left (known as the Ponzo illusion). The top white line looks larger because we are using the scale of its background to judge distance and perspective. The two lines are actually identical. The illusion is said to arise because we are applying the normal size constancy mechanism in an inappropriate situation. Figure 11 The Ponzo illusion Relevance Knowing how large signals are, the size of the image that a signal casts on a driver s retina should enable him or her to assess how far away the signal is. (D) Similarly, familiarity regarding the brightness of a signal aspect, signal positioning and spacing would provide the driver with distance cues. (D) The lack of visual cues present in dark conditions, for example, in a tunnel makes it more difficult for a driver to accurately judge speed and braking distance. (D I) RAIL SAFETY AND STANDARDS BOARD 69

72 Page 70 of Implications Incorrect signal intensity can exacerbate read-ahead, read-through, or readacross if drivers misinterpret brighter signals to be nearer. (I) Uneven signal spacing confounds driver expectation and can result in signals being missed, or braking distances being misjudged. Driver s route knowledge should, in part, provide some mitigation against this problem. (D I) The distance of wrong side signals may be more difficult for drivers to judge. Clause C1.2.2 of specifies that signals can be positioned on the right-hand side of the line, but only if a number of mitigating conditions are present. (D I) Clause C1.4.1 of states that successive signals for the same direction of traffic flow shall be reasonably consistent in form. Consistency in all aspects of signal design should facilitate the driver s ability to accurately assess the distance of signals, and subsequent association of signals with lines. (D) The provision of visual cues in the form of retro-reflective count-down markers in tunnels may be beneficial in enabling drivers to more accurately judge their speed and braking distance to a signal ahead. (D) Motion parallax Motion parallax refers to the relative movement of images over the retina. If two objects are moving left to right across the line of vision at the same speed but one object is much further away, the image cast by the nearest object would move much further across the retina than would the image cast by the more distant object Relevance Motion parallax may cause adjacent parallel signals to appear to change position as the train driver rounds bends on a curved approach. A curved approach to signals can result in read across to the incorrect signal. (D I) Implication Positioning signals on bends should be avoided. If this is not possible, efforts should be made to ensure that parallel signals can all be seen at the same time on approach to prevent read across errors. (D) Pattern matching Humans are very good at matching patterns, and are able to identify any anomalies in a pattern quickly and with precision. By making one part of a pattern different, its conspicuity should be greatly enhanced Relevance Changing a signal s appearance in some way could enhance the speed with which a driver could identify it. This could be beneficial in aiding the identification of multi-spad signals that are positioned on a gantry of three or more parallel signals. (D) Implications Changing the profile of a signal from adjacent parallel signals could be achieved in a number of ways. Modifying the size of the signal backplate is one suggestion; another may be the addition of a line ID plate above the signal. This may be particularly useful in locations where there are a large number of lines and it is difficult to associate a signal with its appropriate line. (D) 70 RAIL SAFETY AND STANDARDS BOARD

73 Page 71 of 71 Staggering signals on a gantry has been the subject of some research. It has been found to be a useful cue under certain circumstances. As long as the full 7+1 seconds of viewing is unobstructed for all signals on the gantry, staggering signals has been demonstrated to aid earlier sighting. However, if these sighting conditions are not met, staggering has been shown to induce read-across errors. (D) Figure 12 Example of signal staggering RAIL SAFETY AND STANDARDS BOARD 71