Motor Vehicle Death Reductions from Guardrail Installation

Abstract

The effectiveness of guardrail installations and other road modifications to reduce injuries has been questioned recently based on critiques of study designs comparing injuries before and after installation at high risk sites. So-called “regression to the mean” and “accident migration” are claimed to greatly reduce previous effectiveness estimates. In this study, fatalities on 161 km of road from the 7 years before and 10 years after guardrail installation at selected sites were compared, noting where installations occurred and how many people died when vehicles went over embankments. Regression analysis was used to estimate the effects of the guardrails, seat belt law, average daily traffic, and trend. In the 10 years after installation, 21 fatalities were expected but no fatalities occurred at new guardrail installations, an average reduction of about two per year. Other factors were not significant. Based on fatalities on the interspersed sections of road with no new guardrail, there is no evidence of “regression to the mean” or “accident migration.”