Study of the Impact of Police Enforcement on Motorists’ Speeds

One method to enforce speed limits on U.S. highways is through the use of police enforcement. One major problem with this approach is the high cost associated with the employment of police officers for patrolling purposes. Thus, the effectiveness of police presence in controlling highway speeds should be carefully assessed prior to making a decision on increasing police patrolling operations on the transportation network. A study was undertaken to evaluate the effects of police presence on the speed of vehicles in a recently increased speed-limit zone and to determine the “halo” effects of police presence (lasting effects when police are gone) on vehicular speeds. Data were collected on Interstate 96 in Ionia County, Michigan. Marked police cars circulated in both directions on the study segment, which was approximately 45 km (29 mi) long. Net speed reductions at a distance of 1.6, 3.2, and 4.8 km (1, 2, and 3 mi) upstream and downstream of each counter were used in the data analysis. Two methods were applied to study the effects of police presence. First, the average speed in the area without the police presence was compared to the average speed after the police presence. Second, the difference between the speed at the place where the police vehicle was positioned and the speed downstream was calculated. It was found that the average speed just upstream of the police car’s location was reduced, but as soon as vehicles passed the patrol car, drivers accelerated to their normal speeds or more. Moreover, no “halo” effects on the vehicles at the increased speed zone under study were observed.