Signal Passed At Danger

A signal passed at danger in railway terminology of many countries, including Australia and the UK (where it is also known as either a SPAD or SPAR), occurs when a train passes a stop signal without authority to do so. The United States's NTSB, which investigates railway accidents in that country, typically terms this as running a red signal.

Because it takes a considerable distance to bring a train to a stand, many signals are passed at danger at low speed where the driver has applied the brake too late. Often this occurs when the signal at danger cannot be clearly seen until it is closer than the braking distance of the train. It can also be due to:

In some situations, however, the driver is unaware that they have passed a signal at danger and so continues until a collision occurs, as in the Ladbroke Grove rail crash. In this instance, it is up to the safety system (where fitted) to apply the brakes, or for the signaller to alert the driver.

Automatic train protection (ATP) is a much more advanced form of Train Stop, which can regulate the speed of trains in many more situations other than at a stop signal. ATP supervises speed restrictions and distance to danger points. An ATP does take into account individual train characteristics such as brake performance. Thus, the ATP determines when brakes should be applied in order to stop the train before getting beyond the danger point. In the UK, only a small percentage of trains (First Great Western and Chiltern Railways) are fitted with this equipment.

The DRA is an inhibiting switch located on the driver's desk of United Kingdom passenger trains designed specifically to prevent 'Starting Away SPADs'. The driver is required to operate the DRA whenever the train is brought to a stand, either after passing a signal displaying caution or at a signal displaying danger.

Once applied, the DRA displays a red light and prevents traction power from being taken.

Whilst the ideal safety system would prevent a SPAD from occurring, most equipment in current use does not stop the train before it has passed the Danger signal. However, provided that the train stops within the designated overlap beyond that signal, a collision should not occur.

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