intermittent and continuous ATP 101119 - irse.nl

contradiction stems from the fact that where previously an ideal driver may havebeen assumed in signal layouts, the introduction of ATP usually is the consequenceof a more explicit risk evaluation concerning train and driver failure modes, combinedwith the requirement to prevent these risks causing accidents to a much higherdegree of certainty. Secondly of course the introduction of technical systems willalways introduce trade-offs between response times, availability, delays caused byspurious or erroneous interventions etc. Technology will almost always be lessadaptive and flexible, especially in the case of vital systems, and limit the humanoperator’s degrees of freedom. On top of that, in the design, as in any technicalsystem, (safety)margins are introduced and enforced.ATP enforces line speeds and in most modern systems supervises brake curves.That limits a driver’s ability to make up time.Defensive driving techniques (with or without ATP) involve. not entering theplatform at maximum speed and coming to a stop within 1 m of the overlap ordeparture signal with screeching brakes etc. ATP enforces this.Rolling stock engineers asked to specify guaranteed brake performance of theirtrains “to SIL 4 requirements” will incorporate their own safety margins. Engineersdevising slip/slide algorithms required in position determination as part of brake curvesupervision, will try to err on the safe side etc.In this chapter we examine the capacity effects and a number of strategies tomitigate them or even improve line capacity using ATP/ATC systems. In practice theirefficiency would depend on the proportion of the rolling stock being equipped with therequired ATP/ATC capabilities.Intermittent SystemsIn very simple intermittent ATP systems such as AWS or Indusi, the train-stopsystem only applies the emergency brake when the driver does not acknowledge awarning or passes a signal at danger. Therefore an overlap is required to allow atrain to be stopped within the distance to the danger point that is being protected bythat signal. In most of those systems a (standard) overlap is used, which for exampleis nominally 200 yards in the BR system. This overlap reduces line capacity, althoughof course that overlap would also have to be required and perhaps even have to belonger, on a non-ATP railway.In German signalling systems, the overlap is only protected when the train isapproaching the signal and is still at a considerable distance. Depending on timeelapsed or position of the approaching train, the overlap can subsequently bereleased. This certainly improves the headway of trains, but on the other hand itincreases the danger of an accident when the overlap has been released and thetrain is erroneously accelerating again.Brake Supervision And Release SpeedIn ATP systems that check if the brakes are applied after receiving a “caution” or“prepare to stop” type of command from the trackside, the driver can no longerimmediately react on an improvement of the signal aspect and release the brake oraccelerate as he would have been able to do in the absence of ATP. To compensatefor this some systems allow the driver to override, or release, this brake supervisionafter observing the signal aspect ahead improving. This of course has to be weighedagainst the risk of errors of perception etc as this override can easily become a reflexdue to its repetitive nature especially on densely trafficked lines.If supervision of braking curves is implemented, the brake can be released eitherwhen the speed is below a predefined value, or the driver has taken special a action,6