Development of traffic and traffic safety: global trends and incidental fluctuations.

In this, and previous related papers, developments of traffic volumes and traffic safety are studied from a system-theoretic point of view. It was proved already that such an approach is effective and leads to new and interesting theoretical facts. Models for the description of long-term developments of traffic volumes and safety were established, and particular relations between both developments were found. One of these findings was, that the number of fatalities turned out to be a function of the derivative of the logistic function describing the development of the number of vehicle kilometres over the years. It was noted that the model deviations for the number of fatalities and the vehicle kilometres were not merely random, but also had systematic components. In the present paper these deviations were further investigated. For the Netherlands, it is clearly shown that the number of fatalities is a function of the derivative of the number of vehicle kilometres, but with a shift in time. The short-term deviations from this long-term development are also related to the derivative of the number of vehicle kilometres, but without a shift in time. For five other countries similar relations were found, although the description of the short-term fluctuations seems to be a bit more complicated. In general, it can be concluded that the system-theoretic approach turned out to be highly effective for the description of long-term and short-term developments in traffic volumes and safety. For the Netherlands up to 95% of the variance in fatalities can be "explained" by developments in traffic volumes alone. It is clearly demonstrated that, although it will be very difficult to single out effects of specific safety measures, the combined safety actions taken by a society are very effective in getting the safety factor under control. The most valuable fact that can be learned from this analysis is that, if increased developments in traffic volume occur or are predicted, the immediate safety effects can also be estimated. Therefore, anticipatory safety measures can be taken to cope with these effects before (or immediately after) the moment they appear, improving safety management at a more rational and preventive basis.