The frequency and severity of road traffic accidents investigated on the basis of state space methods.

The developments in the frequency and severity of road traffic accidents and casualties in Belgium in 1974 - 1999 were investigated. The time series is described in order to discover the trend, quantify the impact of explanatory variables - laws, weather and economic conditions - and predict the accident data of 2000 and 2001. The methodology used throughout the analysis is State Space Methods. The data used in this study are monthly observations from January 1974 till December 1999. In addition to four dependent traffic related variables - the number of accidents with persons killed or seriously injured, the number of accidents with lightly injured persons, the number of persons killed or seriously injured and the number of lightly injured persons - The effect of sixteen independent variables was studied. When apart from the descriptive objective, the explanatory objective is aimed at, the effect of 16 independent variables is tested. Several laws - especially these concerning the mandatory seat belt use in front seats (June 1975), a speed limit of 50 km/h in urban areas (January 1992) and a 0.50/00 blood alcohol concentration (December 1994) - had a clear positive effect. Apart from those, the weather elements precipitation, sun, frost and thunderstorm were important. Frost was the only weather condition with a favorable effect on traffic safety. On the subject of the economic variables, the number of unemployed and the number of car registrations seemed significant. The models developed in this text show large potential for describing the long term trends in traffic safety. On the one hand, they can isolate the effect of phenomena that cannot be influenced, but certainly act upon traffic safety (for example the weather). Similarly, macro-economic and socio-demographic evolutions could be added to the model. On the other hand, the efficiency of policy decisions (for example laws) can be tested. These are the direct tools for increasing the level of traffic safety. Apart from quantifying the impact of variables, forecasts can be made and the corresponding uncertainty can be estimated. For the covering abstract please see ITRD E135207.