, Professor, Civil, Environmental and Geo-Engineering
This project compared the behavior of left-turning drivers with green ball permitted left-turn indications versus those with flashing yellow arrow indications. Researchers hypothesized that drivers with the green ball indication were more likely to attempt turns without stopping, more likely to attempt shorter gaps, and more likely to cause deceleration by opposing drivers than were left-turners with flashing yellow arrows. These were investigated with a field study of driver behavior with the two conditions. A digital video camera was used to record left-turning vehicles and through vehicles at an urban intersection. A total of 39 left-turn events, with 195 gap decisions, were identified, and vehicle trajectories corresponding to those were extracted from the video and transformed into real coordinates using photogrammetry. Bayes estimates of each opposing vehicle's distance, speed, and time-to-arrival were then computed from the trajectories and used as predictors in logit models of acceptance/rejection decisions. It was found, when models were penalized for the numbers of their parameters, that arrival time--the ratio of initial distance to initial speed was the best predictor. This contrasts with an earlier study that found distance clearly superior to arrival time. This may be due to the fact that in the current study, speeds and initial distances were substantially higher than in the earlier study.