Estimations of the expected effects of changed speed limits on driving speed and crash rate are most often based on data only from road sections where the limits have been changed. Previous research indicates, however, that changing the speed limit on a road section may affect driving speeds even on road sections where the limit remains unchanged. In the present study such effects were investigated on two road sections in Norway. Speed limits were changed from 90 to 80 or from 90 to 100 km/h, and speed measurements were made both on the road sections where the limits were changed and on adjacent sections. In order to assess to what extent speed changes on the adjacent road section can be attributed to the speed limit change, speeds of vehicles coming from the section with changed speed limit were compared to that of vehicles in the opposite direction. On both roads the speed on adjacent sections changed in the same direction as the speed limit change. Although the effects were rather small (12 km/h), they may have significant implications for crash risk. Models for predicting the consequences of speed limit changes should therefore take such effects into consideration. Alternative behavioural explanations are discussed. (a) For the covering entry of this conference, please see ITRD abstract no. 0612AR242E.
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