The influence of belief that a car crashed on witnesses' estimates of civilian and police car speed

The authors performed 2 experiments investigating the influence of the belief that a vehicle crashed on witnesses' estimates of the vehicle's speed. In Experiment 1, participants saw a video of a civilian car being driven, after which they were assigned to 1 of 2 conditions. The 1st group was told that the vehicle subsequently crashed; the 2nd group was not told that the vehicle crashed. The results indicted no differences between the 2 groups on a number of factors, including estimates of the vehicle's speed. Experiment 2 was identical except that the video showed a police car using flashing lights and sirens. Participants who had been told that the car had crashed overestimated speed, the likelihood of a crash, and the likelihood of someone being killed. Participants who were not told that the vehicle crashed estimated the speed of the vehicle accurately. Confidence in their estimates of speed was not significantly different between the 2 groups. Results are discussed with regard to police investigations of road accidents.