The effects of mobile roadside speedometers on traffic speeds

A study was conducted to evaluate mobile roadside speedometers as a means of controlling urban traffic speeds under varied schedules of deployment and speed limit law enforcement. Speeds of cars passing the roadside speedometer were measured using nondetectable radar. The data indicate that, generally, the speedometer's presence reduced average traffic speeds by about 10% alongside the speedometer and about 7% at short distances downstream. The proportion of drivers exceeding the speed limit by at least 10 mph fell dramatically from 15%-20% to only 2% at one site on days the speedometer was deployed, and the device was particularly effective when deployed in school zones. However, the effect of the speedometer was limited to the times when it was actually deployed. Associated police enforcement is a key factor, as the effect of the speedometer decayed over time but could be long lasting with a minimal amount of enforcement activity in the area of the speedometer.