Chapter 18 Before–After Evaluations

Purpose – The focus of this chapter is on state-of-the-art methodology for observational before–after evaluations. The purpose of such evaluations is to acquire information on the safety effects of site-specific treatments that could be used by road agencies for future interventions that may have an impact on safety. Methodology – Information, illustrated by a detailed example, is presented mainly on what is still regarded as the gold standard – the Empirical Bayes (EB) method. The methodology accounts for observed changes in crash frequencies that may be due to regression-to-the-mean, changes in traffic volume and time trends. Two related approaches – a comparison group method and the Full Bayes approach, which have been legitimately used for evaluations under special circumstances – are discussed in brief. Findings – The EB before–after methodology remains the gold standard but must be applied properly. Research implications – Some issues, for example, the selection of reference groups, still require further research. Practical implications – Key issues in the application of the before–after evaluation methods in general are also discussed.