Use of an unmanned police car to reduce traffic speed

Motor vehicle collisions are the most common mechanism of traumatic death. Speeding is often implicated as a causal factor in motor vehicle crashes. One potential intervention, to prevent speeding, is the placement of a roadside unmanned police car. This study sought to answer the following questions: is speeding reduced by this intervention, does this intervention lose effectiveness over time, and when the car is removed, do motorists resume speeding?

A radarless speed detector was placed on a roadway that had a history of speed-related collisions. Baseline speeds were recorded for 12 days. Thereafter, an unmanned police cruiser was parked near the road, and speeds were recorded for 10 days. The police car was removed, and data collection continued 1 more week. The difference between the proportion of motorists exceeding 45-mph in the baseline period and the decoy intervention period was tested by using a chi2 test.

During the baseline surveillance, 72.0% of vehicles (186,578 of 259,074 motorists) had speeds greater than 45 mph. After placement of the unmanned police car, 41.0% of motorists (92,272 of 225,026 motorists) exceeded 45 mph (p < .0001). Over the 10-day study period, when the decoy police car was in place, the percentage of motorists exceeding 45 mph gradually increased from 27.2% to 47.4%. Upon removal of the police car, speeding returned approximately to baseline, with 67.5% of motorists (120,640 of 178,752 motorists) exceeding 45-mph.

Parking an unmanned police car beside a road was associated with a large reduction in speeding over a 10-day period. Removal of the unmanned police car resulted in a return to preintervention speeding.