Evidence from investigation of accidents support the idea that organisational factors are important to SPADs.
Infrastructure factors. Southampton signals had been passed 7 times; such signals are called Multi-SPAD signals. The signal at Ladbroke Grove was also known to be difficult for drivers to see and had been passed at danger before. Recommendations to improve the visibility of the signal existed at the time of the accident.
The density of traffic near Ladbroke Grove was high; it was critical section of track for achieving the level of services the operators wanted. The report into that accident examine issues such as the training of drivers and the improvement of known weaknesses in the signalling system.
Operational factors. The train operators are also subject to timetable pressure. Such pressure can conflict with the driver’s training to drive cautiously. After the SPAD, driver at Southampton breached procedures to move the train in circumstances when this was forbidden. Why?
If it is accepted that organisational factors are relevant what are the consequences? We argue that it is important to model the contribution of the factors to the occurrence of SPADs. Note that this not primarily to assess the overall risk – this is clear from data. Moreover, current methods such as fault trees are not suitable for this.