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 α = 0.01.