Incorporating Traffic Operation Measures in Safety Analysis at Signalized Intersections

Abstract

Level of service (LOS) is applied to measure traffic operation conditions and is used as a performance measure in transportation planning. Safety is an important issue in operation and transportation planning. Although it is generally agreed that operational conditions have an effect on traffic safety, their relationship has not been well defined at a level of detail that adequately supports operation and planning performance measurements. Signalized intersections are the most complex locations on the roadway network and are vital to the efficiency and safety of transportation systems. Based on data collected on 164 four-legged signalized intersections in the state of Florida, this study investigated the relationship between LOS and safety at signalized intersections. The LOS was calculated for three time periods: a.m. peak, midday, and p.m. peak. The generalized estimating equations with a negative binomial link function were applied to fit crash frequency models for total crashes, rear-end and sideswipe crashes, angle and left-turn crashes, and severe crashes for the selected intersections. Among operational measures, the study found that the six-level LOS indicator was better than the average delay in the crash frequency models. Overall, the LOS indicator was a significant factor for predicting crash occurrence at intersections. However, the models showed that the operational condition itself (by only including the LOS indicator) was not sufficient to predict the safety status. LOS D was a desirable level that was associated with fewer total crashes, rear-end and sideswipe crashes, and right-angle and left-turn crashes.