Priming drivers by exposure to active and inactive speed displays

This study examined the effect of Speed Display Signs (SDSs) on drivers’ behavior, using two SDSs positioned two kilometers apart from each other. Driving speeds were recorded under two conditions. In Condition I both SDSs were on, whereas in Condition II the first SDS (SDS1) was shut down and only the second (SDS2) was on. Traveling speeds were compared upon reaching SDS2. Our findings document that in Condition II, compared to Condition I, the average traveling speed, measured at SDS2, was lower by 5 kilometers per hour (km/h) (The results mentioned in the abstract are all with p-value < 0.01); the proportion of drivers obeying the speed limit increased from 3% to 15%; and the proportion of drivers traveling at extreme speeds decreased from 50% to 32%. We interpret the results as follows: In Condition I, drivers are informed of their traveling speed by SDS1, and consequently adjust it to their perceived apprehension threshold (We define apprehension threshold as the maximum speed at which drivers perceive as negligible the probability of being pulled over by the police for violating the speed limit.), which is above the official speed limit. In contrast, when SDS1 is off they are uninformed of their traveling speed, while being primed for potential enforcement down the road; as a result, thus impulsively slow down. A major consequence of our findings is that the relevant authorities must rethink how to use the SDS tool in order to encourage drivers to comply with the speed limit.