The introduction of positive offsets for left-turn lanes is a low-cost countermeasure for improving road safety at signalized intersections. Positive left-turn lane offsets provide better line-of-sight of the opposing. The costs range from a low of $200/approach to a high of $15,000/approach. Researchers have reviewed the safety benefits using empirical Bayesian before-and-after studies and have found that there exists positive safety impacts through the reduction of rear-end and angled crashes. However, these studies did not assess whether the presence of the countermeasure and its interaction with various traffic factors, such as left-turn movement volumes and truck percentages, had statistically significant effects due to the lack of available data. A more mechanistic methodology is needed to assess the safety benefits this countermeasure. Microscopic simulation models provide a basis for this mechanistic approach since they are governed by underlying psycho-physical driver behavioural models. The integration of simulation and surrogate safety performance measures, such as time-to-collision, allows for the assessment of safety benefits in lieu of observational crash data. A factorial experiment was used to assess the statistical significance of main effects and interaction effects. It was found that the main effects of the countermeasure, major AADT, minor AADT, and left-turn lane volumes had statistically significant impacts on total, merging, rear-end, and lane-change conflicts. The presence of the countermeasure improved safety for all types of conflicts. This study provided practitioners insight that specific traffic-related factors must be considered when evaluating the implementation of positive left-turn lane offsets. For the covering abstract of this conference see ITRD number E220308.
Abstract