Crash Modification Functions for Installation of Left-Turn Lanes at Signalized Intersection Approaches

This paper presents the results of a study that developed crash modification (CM) functions for installing left-turn lanes at signalized intersection approaches. CM functions were obtained from a longitudinal before after safety study that accounted for treatment location characteristics (heterogeneity). This approach for developing CM functions has several advantages over the commonly used cross-sectional evaluations, which have several statistical shortcomings. The developed CM functions incorporate a time variable to acknowledge that the safety treatment effects do not occur instantaneously but are spread over future time; this result was achieved with a nonlinear intervention model with the full Bayes method. Twelve treatment sites were selected for the evaluation, along with 67 comparison sites. The treatment included the addition of one or more LT lanes for each intersection. The analysis showed significant safety improvements for fatal-plus-injury and total collisions but statistically nonsignificant reductions for property-damage-only collisions. The significant covariates included in the CM functions were the time trend, total entering volumes, and category of the new left-turn lanes installation.