AN EXPERIMENTAL-STUDY TO EVALUATE THE EFFECTIVENESS OF DIFFERENT METHODS AND INTENSITIES OF LAW-ENFORCEMENT ON DRIVING SPEED ON MOTORWAYS

Two field experiments were conducted to establish the most effective m ethod of enforcement in reducing driving speed and to establish the mo st efficient strategy in terms of police force personnel required. In the first experiment, the effect of three variables on driving speed o n motorways was studied. The first variable, intensity of enforcement, was manipulated by creating three different objective levels of appre hension for detected speeding drivers. The second variable was method of enforcement. On-view stopping and ticketing of offenders was compar ed to mailing of fines on the basis of the car's licence plate number. The third variable manipulated was the time delay in the mailing of f ines. Time delay between detection of the offence and feedback to the driver was shortened in one condition. In a second experiment, police enforcement effort was optimized by relating intensity level of enforc ement to the proportion of speeding vehicles. In the first experiment, police enforcement was applied for four weeks; in the second experime nt, enforcement activities were continued for 12 consecutive weeks. In both cases the main dependent variable was driving speed. Before, dur ing, and after the application of enforcement, speed was registered us ing induction loops. In addition, driver opinion about speeding and sp eed enforcement was studied using postal questionnaire surveys. The re sults show the largest and longest lasting reduction in driving speed in the highest intensity level-condition, giving support for a relatio n between objective chance of apprehension and speed choice. On-view s topping of offenders was shown to be a more effective method to reduce driving speed than mailing of fines. Some of these results are discus sed in the light of game theory. The questionnaire surveys indicated t hat most drivers did not notice the recurrence in enforcement activiti es due to infrequent passing of the sections of motorways studied. The preventive effect of police enforcement appeared to be far more subst antial than its repressive effect. Enforcement primarily deters the cu rrent nonoffender from speeding.