An exploratory analysis of models for estimating the combined effects of road safety measures

Road safety programmes consisting of a large number of road safety measures have been developed in many countries. To estimate the effects of such programmes on the number of accidents, models for estimating the combined effects of road safety measures are needed. This paper presents an exploratory analysis of such models. There is very little empirical evidence to support model building. Based on a few studies that have evaluated the effects of multiple road safety measures introduced at the same locations, the paper compares two models. One of the models, the common residuals model, assumes that the (percentage) effect of a road safety measure remains unchanged when it is combined with other road safety measures. The other model, the dominant common residuals model, assumes that the most effective measure in a set of measures has a dominant effect that weakens the effects of other road safety measures it is combined with. Evidence from the few studies that were found is consistent with both these models. A study of the effects of a road safety programme implemented in Victoria, Australia between 1990 and 1996 indicated that the effects of safety measures are weakened when these measures are combined with other road safety measures.