This study sets out to identify and quantify those factors which influence a driver's choice of speed. The factors investigated included demographic variables, driving habits, accident histories, and a range of psychological variables. The speeds and registration numbers of free-flowing vehicles were recorded on video. Using the registration number information, self-completion questionnaires were sent to the relevant drivers in order to collect detailed information on the variables under investigation, and to relate this to the speeds at which the drivers were travelling when filmed. Information was collected from over 5000 drivers at 24 differerent sites. Of those observed, the faster drivers tended to be young, driving high annual mileage in large cars, and to be travelling alone to and from work. A variety of interacting factors were found to influence a driver's choice of speed, but the largest single influence was from site characteristics, which accounted for over half of the variation in speed. Because of this, the analysis concentrated on speed relative to the mean for each site, rather than the absolute speed. Multivariate techniques were use to analyse the speed data and to explore the contribution of the demographic and psychological variables. Of the former, age proved to be the strongest predictor, and some of the psychological variables turned out to be reasonably robust. The modelling of the accident data confirmed results from recent studies, and the analysis suggests that a 1% change in the speed choice of an individual driver is associated with a 7.75% change in accident liability. (A)

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