Exceeding the speed limit: an evaluation of the effectiveness of a police intervention.

Driving too fast is probably one of the main contributors to the occurrence and severity of road accidents, and intention to speed is an important predictor of exceeding speed limits. This study examined the effect of a police intervention on exceeding the posted speed limit (speed of vehicles on the target road) and on intentions to speed (attitude questionnaire). The intervention consisted of a week in which 'police speed check area' warning signs were put up on the target 40 mph limit road, followed by a week of active police presence, followed by a further week in which the signs remained. The speed of vehicles was measured using police data collection equipment for a total of 7 weeks. The effect on the intentions of drivers using the road to exceed speed limits was assessed across age, sex and pre-intervention speeding behaviour using questionnaire measures. Fewer people broke the speed limit during the intervention than before, this effect lasting to a limited extent up to 9 weeks after police activity ceased and 8 weeks after signs were removed. The effect on drivers breaking the speed limit by large amounts was more transient. The intervention reduced intention to speed for subject groupings with high pre-intervention intention. Traffic flow contributed significantly to the variance in vehicle speed, but was not responsible for differences between the weeks, which may therefore be attributed to the intervention. Traffic flow also did not account for differences in speed between the two directions of traffic, which may therefore be attributed to the fact that the road areas preceding the target area in each direction differed in their speed limits (70 vs 30 mph). Intentions to speed, as well as speed adaptation difficulties are though to contribute to these differences.