ZEW Economist Martin Kesternich on the Speed Limit on Motorways

The debate about the speed limit on German motorways has so far been guided mainly by emotional rather than objective arguments. What is missing is a careful examination of the causal effects of a speed limit. The German automobile association ADAC has now also spoken out in favour of determining the effects of a speed limit on the basis of reliable research results. Professor Martin Kesternich, deputy head of the Research Department “Environmental and Resource Economics, Environmental Management” at ZEW Mannheim, explains what an optimal study could look like using a randomised field experiment to identify the causal effects of a speed limit on road safety.

For a randomised field experiment, a temporary speed limit would have to be introduced on a sufficiently large number of randomly selected motorway sections over a longer period of time. The speed limit would at best apply for a whole year in order to include all seasonal weather conditions in the study. The randomly selected road sections would have to be compared with a control group, i.e. with a corresponding number of comparable motorway sections on which no speed limit applies during the period under study.

In order to determine the optimal speed limit, the speed limits introduced should vary and be precisely defined in advance. By comparing the regulated sections with the non-regulated sections, causal conclusions can be drawn – provided that without regulation traffic safety conditions on the two sections would have developed similarly.

Such a study could be carried out with relatively little effort. Of course, it must be ensured that sensitive data is made anonymous to prevent any inference to individual persons or households.”