The stability of long-term trends in the number of traffic fatalities in a sample of highly motorised countries.

This paper examines the stability of long-term trends in the number of traffic fatalities in eight highly motorised countries: Norway, Sweden, Denmark, Finland, The Netherlands, Great Britain, Australia and The United States. In all these countries, the number of traffic fatalities reached a peak around 1970-1972 and has since declined. The decline has, however, been irregular and fairly long periods of stagnation or even increase in the number of fatalities have occurred in all countries. A stable trend is defined in this paper as a trend that remains unchanged and therefore can be used to successfully predict the future number of fatalities by means of extrapolation. It is concluded that the trends towards fewer fatalities in the countries selected have not been stable and that even trend lines that fit past trends very closely are usually worthless for predictive purposes. An attempt was made to identify factors influencing long-term trends by fitting negative binomial regression models to fatality data. Although some of the models fitted the data marginally better than simple trend lines, these models are not necessarily more useful for predicting future trends than simple trend lines, since using the models for prediction requires that future changes in all explanatory variables must be predicted. The chief lesson is that past trends do not provide a reliable basis for predicting future developments with respect to the number of traffic fatalities.