80 Kilometer zones: Safer before or after?A closer look at the impact of a speed limit reduction on four Dutch motorways

This thesis discusses the combined impact of a reduction from 100 km/h to 80 km/h and permanent speed enforcement on accidents, for four Dutch motorways. Chapter 1 introduces the main question: “What is the impact of the 80 kilometer zones policy on road safety?” and is answered in Chapter 5. Current literature regarding speed, road design and speed enforcement is reviewed and analysed in Chapter 2. The chapter points out the added value of using control groups to correct for trends and regression and two stage least squares analyses to improve statistical validity of the results. Chapter 3 notes that Difference-in-Differences is rejected as a method due to violation of permanent trends assumption and presents regression and two stage least square analysis as valid tools to analyse the relationship between speed and accidents. Chapter 4 presents the results from the regression and two stage least square analysis. The reduction in speed limit reduced the speed and the amount of accidents. An additional effect of enforcement was not found. As speed is a significant factor affecting accidents, Chapter 5 concludes with the final outcome: The introduction of the policy helped reducing speed and accidents and did not cause any spill over effects. Increased speed enforcement help to improve traffic flow, contrary to previous literature, camera’s itself did not affect accidents, only speed. Future research could test if the observed results hold in general, irrespective of road type and surrounding.