Signal passed at danger


A signal passed at danger, known in the United States and Canada as running a red light, is an event on the railway where a train passes a stop signal without authority. Where colour light signals are in use, a S.P.A.D. occurs when a train passes a red signal without authority, and where semaphore signals are used, a S.P.A.D. occurs when a train passes a signal in the 'on' position without authority.
In the U.K, the alternative description signal passed at red is used where a signal changes to red in front of a train due to either a technical fault or in an emergency, such that the train is unable to stop before passing the signal, despite being driven correctly.

Causes

It takes a considerable distance to stop a train, and incidents of this type often involve a slight or very slight overrun of the signal, at low speed, because the driver has braked too late, often after sighting the signal too late.
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 such cases it is up to the safety system to apply the brakes, or for the signaller to alert the driver.
Some of the causes are:

Automatic train protection

Automatic train protection 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 will also take into account the individual train characteristics such as brake performance. Thus, the ATP determines when brakes should be applied in order to stop the train before getting to the danger point. In the UK, only a small percentage of trains are fitted with this equipment.

Driver's reminder appliance

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.

Collision prevention systems

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.

Train stops

On the London Underground, mechanical train stops are fitted beside the track at signals to stop a train, should an S.P.A.D occur.
Train stops are also installed on main line railways in places where tripcock equipped trains run in extensive tunnels, such as the on the Northern City Line where the Automatic warning system and Train Protection & Warning System are not fitted.

Train Protection & Warning System

On the UK mainline, TPWS consists of an on-board receiver/timer connected to the emergency braking system of a train, and radio frequency transmitter loops located on the track. The 'Overspeed Sensor System' pair of loops is located on the approach to the signal, and will activate the train's emergency brake if it approaches faster than the 'trigger speed' when the signal is at danger. The 'Train Stop System' pair of loops is located at the signal, and will activate the emergency brake if the train passes over them at any speed when the signal is at danger.
TPWS has proved to be an effective system in the UK, and has prevented several significant collisions. However, its deployment is not universal; only those signals where the risk of collision is considered to be significant are fitted with it.

Flank protection

At certain junctions, especially where if the signal protecting the junction was passed at danger a side collision is likely to result, then flank protection may be used. Derailers and/or facing points beyond the signal protecting the junction will be set in such a position to allow a safe overlap if the signal was passed without authority. This effectively removes the chance of a side-impact collision as the train would be diverted in a parallel path to the approaching train.

SPAD indicators

Prior to the introduction of TPWS in the UK, "SPAD indicators" were introduced at 'high risk' locations. Consisting of three red lamps, they are placed beyond the protecting stop signal and are normally unlit. If a driver passes the signal at 'danger', the top and bottom lamps flash red and the centre lamp is lit continuously. Whenever a SPAD indicator activates, all drivers who observe it are required to stop immediately, even if they can see that the signal pertaining to their own train is showing a proceed aspect. Since the introduction of TPWS, provision of new SPAD indicators has become less common.

UK acronyms: SPAD / SPAR

In the UK, incidents where a signal is passed at danger without authority are categorised according to principal cause. A SPAD is where the train proceeds beyond its authorised movement to an unauthorised movement. Other types are categorised as SPAR.
Prior to December 2012, the term "SPAD" applied to all such incidents, with a letter specifying cause.
SPADs arising on starting from stations - typically those informally termed 'ding-ding right-aways', where the driver has inadvertently acted on a guard or dispatcher's indication without reading the signal - can be classified further:
Signals form part of a complex system, and it is inevitable that faults may occur. They are designed to fail safe, so that when problems occur, the affected signal indicates danger. To keep the network running, safety rules enable trains to pass signals that cannot be cleared to a proceed aspect. Provided that authority for the movement is obtained, a SPAD does not occur. There are two methods of obtaining that authority:

Driver obtains signaller's authority to pass a signal at danger

Once the train has been brought to a stand at a signal which is at danger, the driver should attempt to contact the signaller. If the signal cannot be cleared then the driver must obtain the signaller's authority to pass it at danger. Methods for contacting the signaller may include GSM-R cab radio, signal post telephone or mobile phone.
The signaller can authorise a driver to pass a signal at danger when:
The driver and signaller must come to a clear understanding, and ensure they agree about how it is to be done. In the UK the signaller tells the driver of a specific train to pass a specific signal at danger, proceed with caution and travel at a speed that enables him to stop short of any obstruction, and then obey all other signals. If the signal is fitted with TPWS, the driver resets the Driver Reminder Appliance, pushes the TPWS Trainstop Override button in the cab, and proceeds cautiously through the section. If the train reaches the next signal without finding an obstruction, he must obey its aspect, at which point he can revert to normal working.

Driver passes a signal at danger under his own authority

If contact with the signaller cannot be made then the driver must not move the train, unless it is standing at one of the following signals:
After passing a signal at danger under his own authority, the driver must stop at the next signal and inform the signaller of what he has done.

Accidents involving a signal passed at danger without authority

Whenever a signal is passed at danger the driver is required to "proceed with caution, stop short of any obstructions, and drive at speed that will enable you to stop within the distance which you can see to be clear". Failure to do this has caused the following collisions:
Except where permissive working is in use, interlocking usually prevents a train from being signalled into a section that is already occupied. When operational needs require it, this can be overridden, and provided it is carried out in accordance with the rules this is a safe practice. However, failure to follow protocol can result in a collision: