This paper describes an investigation into the safety effects of all types of engineering schemes implemented on rural and suburban highway networks in twelve local authority areas: some 2300 schemes in all. The effects of treatment are estimated in two ways: using control sites and using a simple graphical time series analysis. With a view to forming the basis of a method for allowing for regression-to-mean effects without the use of controls, the accuracy of the default accident frequencies recommended by the Department of Transport for use in its computer cost-benefit analysis program COBA is investigated. The results indicate that, overall, the highway engineering schemes resulted in rather modest accident reduction although, since the majority of sites were not high risk sites, and the majority of schemes were not accident remedial schemes, the results were not conclusive regarding the reductions achievable at such sites. No migration effects were detected. The quality of the COBA estimates varied with link length, with very large errors for short links. (A)
Abstract