Study on left turn safety and gap acceptance at unsignalized intersections

Date of Completion

January 2009


Engineering, Civil




The dissertation consists of three studies. The first study investigates how safety is related to the presence of left turn lanes at unsignalized intersections. Crash prediction models were estimated using crash and volume data from intersections without left turn lanes by intersection category and crash category. Negative binomial modeling was used with generalized estimation equations to account for the correlation among the crash counts for an intersection throughout the years. The expected number of crashes was predicted for each intersection with left turn lanes and compared with the observed crash counts. The results show that when left turn lanes are present, same direction crashes are significantly lower for all intersection categories except the intersections with two lane major road at urban area, for which the safety effects are not discernable. It is also found that the presence of left turn lanes can dramatically reduce fatal and injury crashes at intersections in rural areas. ^ In the second study, field studies were conducted to study the gap acceptance behavior of left turning drivers at six unsignalized intersections. Correlated logit models were used to estimate the probability of accepting a given gap and the best model was selected by the quasi-likelihood information criterion. The results show that the number of lanes on the major road, the presence of left turn lanes, and the gender of the driver are significant in explaining the variation of the gap acceptance probability. Gap acceptance probability functions were determined by intersection type. In the third study, to focus on elderly drivers' gap acceptance behavior, data were extracted from two intersections, where a sufficient number of elderly drivers were observed. Parameter estimates of the correlated logit model show that drivers above age 70 are less likely to accept shorter gaps, suggesting that they compensate the diminished perception and reaction ability by taking a more conservative driving attitude. However, drivers between 55 and 70 years of age show no significant differences in gap acceptance behavior compared to younger drivers. Finally, traffic simulations were run in CORSIM with gap acceptance distributions representing different driver compositions to determine how the age differences could impact traffic operations. ^