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A collision or road run-off occurs where the demands of the driving task exceed the driver’s capability and the driver loses control of the task. There are many determinants of the level of task demand on the driver, some very remote (such as choice of vehicle) and some less remote (such as choice of route). In contrast, others are very immediate, such as the actions of other road users. However, the key determinant of task difficulty, which is under the direct control of the driver, is speed. Speed is important because it determines the ‘time window’of opportunity for the driver to detect and recognise what is going on, decide what to do about it and to execute any action decisions (see review by Aarts and van Schagen, 2006).

High speed means a short ‘time window’and, if it is too short, it will push the task demand beyond the capability of the driver. Thus level of speed is associated with crash frequency, a relationship which is described as either a power function (Maycock et al., 1998; Quimby et al., 1999) or an exponential function (Fildes et al., 1991; Kloeden et al., 1997, 2001). Therefore those who drive fast are more likely to have been recently involved in a crash (Lassarre and Stradling, 2005). Level of speed is also associated with crash severity, which tends to increase exponentially with vehicle speed (Federal Highway Administration, 1998).