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.
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 govern 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 presences 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.
