A comprehensive approach to evaluation of road safety policy

Abstract

This paper outlines a comprehensive approach to the evaluation of road safety policy. An evaluation of road safety policy aims to estimate its effect on the number of traffic fatalities or the number of injured road users. The following main stages of such a study are identified: (1) Analysis of long-term trends for the purpose of developing hypotheses about the effects of road safety policy; (2) Identification of variables describing road safety policy; (3) Identification of confounding variables; (4) Exploratory analysis of statistical models; (5) Comparative analysis of statistical models; (6) Estimation of policy effect and its uncertainty. The approach is illustrated using data for Sweden for 1981–2018. Four variables describing road safety policy were assessed. Only one of them, the length of motorways and 2+1 roads, had a consistent statistical relationship to the number of fatalities. Three models for statistical analysis were compared: a negative binomial regression model, a multivariate ARIMA time-series model, and a least squares linear regression model. The time-series model was clearly the best of the models in terms of various criteria for model quality. According to this model, the number of fatalities in 2018 was 27.6% lower than it would have been without the contribution of the policy variable. It is likely that this estimate is too low. Only a single variable was used as an indicator of road safety policy. The trend term (year count) probably captures part of road safety policy, like the effects of safer cars associated with the renewal of the car fleet. The analyses show that road safety policy in Sweden, as indicated by motorway length, has become more effective after the adoption of Vision Zero than it was before the adoption of Vision Zero. In general, the history of road safety policy cannot be reconstructed in sufficient detail to support an evaluation of which elements of it have been more or less effective. It is, accordingly, not possible to identify any specific set of road safety measures that should be given higher priority in order to make road safety policy more effective.