This paper 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 leg, since 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. Crash prediction models were estimated using crash and volume data from intersections without left turn lanes by intersection category and crash 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. Model assessment suggested that only models for thesame direction crashes provided good model fit. Further study was required to improve the other models before they could be used for prediction. The results of comparing the observed and predicted crashes showed that the intersections were safer for same direction crashes when left turn lanes were installed, except for those on urban two lane roads, at which no safety effect is discernable.
Abstract