Operational and Safety Effects of U-Turns at Signalized Intersections

In an effort to balance vehicular safety, mobility, and access, many transportation officials favor the use of raised median cross sections on highways. However, this decision leads to much controversy from those opposed to the resulting lack of direct driveway access. One of the issues in this controversy is the effect of increased U-turns at adjacent intersections. The purpose of this research was to determine the operational and safety effects of U-turns at signalized intersections. The operational analysis involved measurements of vehicle headways in exclusive left-turn lanes at 14 signalized intersections. Regression analysis of saturation flow data showed a 1.8% saturation flow rate loss in the left-turn lane for every 10% increase in U-turn percentage and an additional 1.5% loss for every 10% U-turns if the U-turning movement was opposed by protected right-turn overlap from the cross street. The safety analysis involved a set of 78 intersections. Fifty-four sites were chosen randomly, and 24 sites were selected on the basis of their reputation as U-turn problem sites. Although the group of study sites was biased toward sites with high U-turn percentages, the study found that 65 of the 78 sites did not have any collisions involving U-turns in the 3-year study period. U-turn collisions at the remaining 13 sites ranged from 0.33 to 3.0 collisions per year. Sites with double left-turn lanes, protected right-turn overlap, or high left-turn and conflicting right-turn traffic volumes were found to have a significantly greater number of U-turn collisions.