Headway Acceptance Characteristics of U-Turning Vehicles at Unsignalized Intersections

During the past two decades, more and more state departments of transportation have installed nontraversable medians and directional median openings on multilane highways. These installations have resulted in an increased number of U-turning vehicles at unsignalized intersections on multilane highways. The headway acceptance characteristics of U-turning vehicles are analyzed at unsignalized intersections on four-lane divided roadways. The critical headway for U-turn movement is estimated by using the maximum likelihood method. A binary logit model is developed to identify the influence of median width on the headway acceptance characteristics of U-turning vehicles. Results of this binary logit model indicate that the median width at a median opening significantly affects the decisions of U-turning drivers about accepting or rejecting a particular headway in the major-street traffic stream. Results of the maximum likelihood estimation indicate that a vehicle making a U-turn at an unsignalized intersection with a wide (=21 ft) median has a smaller critical headway than one making a U-turn at an unsignalized intersection with a narrow (<21 ft) median. The critical headway for U-turn movement at unsignalized intersections is about 6.4 s with wide medians and about 6.9 s with narrow medians. These estimated critical headways supplement the current capacity estimation method for unsignalized intersections and can be directly used in Harders' model to estimate the capacity of U-turn movement at unsignalized intersections.