Influences on road safety development : a study of the applicability of models. Why does the number of road deaths decrease steadily but irregularly? To explain road safety developments by using crash data is not that simple. This is because of a lack of conditions for experimental research. It is just not possible to isolate the effects of a particular measure from other measures and developments. How road safety develops in the course of time is determined by changes in crash rates and exposure. Changes in crash rates are the result of safer vehicles, safer roads, and safer road user behaviour. Changes in exposure are the result of changes in distances travelled and changes in modes of transport. SWOV carried out a study of developments in crash rates as well as exposure. This was done by using techniques of time series analysis. By using models based on these techniques we tried to disentangle the sequence of measures and developments. This report begins with an inventory of existing knowledge about explanatory models and of experience elsewhere with applicable analysis techniques. We then made a number of subdivisions in crash data and formulated hypotheses about road safety development of these crash types in the course of time. Then, for a limited number of crash types and crashes by road type, we carried out time series analyses in order to empirically calculate the influence of certain variables on the crash rates. For this we used a time series analysis technique that is known as state space modelling.
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