The range of replications technique for assessing the external validity of road safety evaluation studies

This paper introduces a simple statistical technique that can be used to assess the external validity of road safety evaluation studies. External validity refers to the possibility of generalising the results of research to other contexts than those in which it was made. There are several aspects of external validity. Two aspects that are often of interest concern the applicability of the results of road safety evaluation studies across countries and time. Can the results of studies made in one or more countries be applied in countries where studies have not been made? Can the results of studies made many years ago still be applied? The technique introduced in this paper is designed to provide support in answering these questions. The technique evaluates the stability of research results in time and space. The technique is based on cumulative meta-analysis and produces statistics that show the consistency of study results in time and space (across countries). The range of replications denotes the span of time and countries in which studies have been made. The idea is that if the results of studies are stable throughout the range of replications, one may have greater confidence in their external validity than if the results of research vary in time and between countries. The technique is illustrated by means of numerical examples.