Safety effects of exclusive left-turn lanes at unsignalized intersections and driveways

This article presents an investigation of the safety effects of exclusive left-turn lane installation at unsignalized intersections. Selected intersections were categorized by area type, the number of lanes, and the number of approach legs, because these characteristics could potentially affect the number of crashes as well as the crash type and severity. Crash data were collected from 1995 to 2004 for each intersection and categorized by (1) same-direction crashes, (2) intersecting crashes, and (3) others. Fatal and injury crashes were also collected. Crash prediction models were estimated using crash and volume data from intersections without left-turn lanes by intersection category. Negative binomial modeling was used with generalized estimation equations to account for the correlation among the crash counts for an intersection through the years. The expected number of crashes was predicted using the prediction models for intersections with left-turn lanes assuming no left-turn lanes were present. If the observed crash counts were significantly lower than the predicted, then there was evidence that the left-turn lane created a safer condition. The results showed that the intersections were safer for same-direction crashes and fatal and injury crashes with some exceptions in urban areas.