A key component of safe railway operations is the adoption of safety management and signalling systems. While these arrangements have progressively developed in sophistication over many years, their effectiveness still relies largely on the human operator to observe procedural requirements and to respond to signal indications appropriately and exactly.
Incidents continue to occur where a train passes beyond a signal at ‘stop’. Although the majority of incidents do not result in damage or injury, for the rail operator, they do result in a loss of process, higher operating costs, and possible regulatory attention. These incidents are also precursors to more serious incidents and accidents.
Although signalling has evolved to higher levels of clarity of indications and track-side positioning, its influence on the train driver may have more of an impact than commonly thought. There can be a ‘contextual uniqueness’ in that a train driver is coerced by the situation in which a signal exists as much as the indication it displays.
The train driving task
The train driving task can be described as routine and with a lack of novelty that promotes an automatic response in which the everyday driving methodologies are used without too much conscious effort.
The issue of boredom in such a routine task as train driving is understandable and as this may contribute to lower levels of concentration, preoccupation with other tasks, however legitimate, may take preference over a response to a signal.
The task of train driving is also a dynamic and decision making one, where a driver needs to plan ahead in preparation of likely events. Humans are also compulsive pattern matchers where a reaction to a situation is from a stock of stored routines.
When assimilating inputs from the driving environment and the requirements of a situation, a train driver will attempt to ‘explain’ a signal indication based on anticipation and previous understanding.
There is of course no support for the notion that train drivers disregard signals.
Responding to a signal
Sustained concentration can be difficult and the integrity of a train driver’s detection and interpretation of a signal can be countered in a number of ways leading to error. Drivers may not routinely fixate on individual signals. The reality is somewhat different as only approximately three seconds will be spent on the task at each location, depending on the circumstances. Once the information has been gleaned and an interpretation acquired, attention may move to other things with perhaps occasional confirmation by a further glance if this is possible. A generous signal sighting distance can therefore be superfluous although the greater the sighting distance, the longer the opportunity to take these three-second and confirmatory glances.
Train driving visual strategies also reveal that there is an expectation of an indication that coexists with looking at a signal. A series of green signals may be seen by a train driver as requiring little attention. The appearance of a caution indication may also not immediately elicit the desired action as there may be an attempt to formulate an explanation for it. This explanation can be influenced by the context under which the train is operating, for example, following another train or coming up to an approach cleared signal.
Interpretation and actions can also be influenced by a signal that does not possess a full range of aspect sequencing or indications, allowing a train driver to determine precisely what the next signal requires. Ordinarily, a caution signal is a warning of a stop signal further on, but it may also signify a less restrictive response such as negotiating a junction or entering a siding. Similarly, a series of cleared shunt signals may lead a train driver into developing an impression of a likely destination rather than the next signal being at stop.
A train driver anticipating proceeding beyond a location may also misread or read the wrong signals because these are the signals expected and wanted.
Forgetting the signal
Memory failure is one of the most common errors and is more to do with forgetting to undertake a future action rather than anything to do with the past.
The nature of line-side signalling requires a train driver to commit to memory each signal until the next comes into view. As a train proceeds along the line and between signals, a train driver can be subject to many distractions and while these diversions can be tolerated when travelling on green signals, they may not be without risk when dealing with a caution signal.
The short-term or working memory is volatile and unless a train driver repeats or rehearses the requirements of each signal, it may dissipate; increasing the risk of susceptibility to distraction and leading to habit intrusion. When the driving task has been interrupted following a caution signal, the resumption of ‘clear signal running’ by a train driver may well occur once the distraction has been dealt with and the intended mission forgotten.
Conspicuity of signals
The fail-safe character and other safeguards built into a signalling system exist in comparison to a train driver who could be seen as a weak link. While it is important that a signal displays a clear indication, it is equally important that a train driver correctly interprets it and ensures that the train is contained within requisite operational boundaries.
The effectiveness of the driver-to-signal interface is often only considered in terms of signal sighting attributes. Although clarity and luminance can be optimised by LED or fibre optics, such a signal’s capacity for optimal interpretation and response can be no better than incandescent types.
The argument for improving black-spot signal conspicuity seems to rely on an impression that train drivers will be able to observe and react to a stop signal in enough time to bring the train to a stop. While the greater the sighting distance of a red signal is, the greater the chances are of it being detected, the origins of many overrun incidents are located well before this signal is reached. Any conspicuity gains, therefore, may be marginal unless up-stream issues are also determined and considered.
A less than suitable response
Distraction is likely to be generic to the driving task and appears to be occurring most of the time; to which a train driver may or may not be susceptible. The degree of automation present in train driving suggests that attention to signals can be overridden by other stimuli.
The simplicity and clarity of a signal indication can be undermined by the circumstances in which it is displayed. As signalling context forms a basis on which a train driver may predict possible outcomes, imprecise interpretation can sometimes lead to a train driver continuing under a false belief, despite optimal signal conspicuity and detection.
Habit intrusion in the lead up to a stop signal can be the result of a train driver forgetting the conditions imposed by a caution signal.
The usual outcome from mistaken interpretation or habit intrusion is a train driver adopting a less than suitable response, which ultimately lowers the chances of being able to stop at a signal when it comes into view. As this human error can be a consequence of the influences of the operational environment, a train driver can be led to act outside the intent of signalling system requirements and not realise it.