The safety value of guardrails and crash cushions: a meta-analysis of evidence from evaluation studies.

Evidence from 32 studies that have evaluated the safety effects of median barriers, guardrails along the edge of the road, and crash cushions (impact attenuators) is summarized by means of a meta-analysis. Two hundred and thirty-two (232) estimates of safety effects are included in the meta-analysis. The presence of publication bias is tested by means of the funnel graph method. For most subsets of the data, no evidence of publication bias is found. Weighted mean estimates of safety effects are computed by means of the logodds method. Median barriers are found to increase accident rate, but reduce accident severity. Guardrails and crash cushions are found to reduce both accident rate and accident severity. The effects of guardrails and crash cushions on accident rate have been less extensively studied than the effects on accident severity. Current estimates of the effects on accident rate are highly uncertain because of methodological shortcomings of available studies. The effects of guardrails on accident severity are found to be quite robust with respect to study design and the number of confounding variables controlled in each study. In general, random variation in the number of accidents is the most important source of variation in study results.