Nilsson's Power Model connecting speed and road trauma: Does it apply on urban roads?

Nilsson (1) proposed power relationships connecting changes in traffic speeds with changes in road crashes at various levels of injury severity. Increases in fatal crashes are related to the 4th power of the increase in mean speed, increases in serious casualty crashes (those involving death or serious injury) according to the 3rd power, and increases in casualty crashes (those involving death or any injury) according to the 2nd power. Increases in numbers of crash victims at cumulative levels of injury severity are related to the crash increases plus higher powers predicting the number of victims per crash. These relationships were empirically derived based on speed changes resulting from a large number of rural speed limit changes in Sweden during 1967-1972. Nilsson (2) noted that there have been very few urban speed limit changes studied to test his power model. Elvik, Christensen and Amundsen (3) meta-analysed 98 evaluation studies from which they derived 460 estimates of the powers relating road trauma changes with travel speed changes, many of which had occurred in urban areas. They found lower powers than those originally proposed by Nilsson, but did not separately analyse the relationships applicable in urban areas. This paper reports a subsequent study based on Elvik et al?s data in which the powers in Nilsson?s model are estimated for urban arterial roads, residential streets, rural highways and freeways separately. It was found that there is no evidence of monotonically increasing powers connecting speed changes on urban arterial roads with increasing injury severity levels. The estimated power applicable to serious casualties on urban arterial roads was significantly less than that on rural highways, which was also significantly less than that on freeways. The implications of these findings for strategic planning predictions of the effects of speed limit reductions are discussed.