Increased police enforcement: effects on speed.

Results of a field experiment in which a 35-km long stretch of road was subjected to an increase in police enforcement--mostly as stationary speed controls--are presented. A group of police officers was invited to plan and perform the enforcement based on their own experience and ideas. The level of enforcement reached a daily average of nine hours throughout an enforcement period of six weeks. Speed measurements were done in 60 and 80 km/h speed-limit zones before, during and after enforcement withdrawal, and were compared to another stretch of road. Average speeds were reduced by 0.9-4.8 km/h in both speed-limit zones and for all times of day. For some time intervals, the average speed and the percentage of speeding drivers were reduced for several weeks of the after-period, demonstrating a time-halo effect of eight weeks at most. The percentage of speeding drivers was reduced in both speed-limit zones for all hours of the day except the morning rush hours 6.00-9.00 A.M. It is suggested that commuting drivers in the morning rush hours are most resistant to speed reduction. These results were statistically significant at alpha = 0.01.