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Estimating Capacity of U-Turns at Unsignalized Intersections

The study presented in this paper is a continuous effort following the authors’ previous studies to estimate the capacity of U-turn movements at unsignalized intersections. The objective of this study is to estimate the capacity of U-turn movements at unsignalized intersections on four-lane divided roads. The study focuses on several issues that have not been addressed in the previous studies. These issues include (a) the impedance effects of minor movements, (b) conflicting traffic volume, and (c) shared-lane capacity of the major street exclusive left-turn lane. To achieve the research objective, traffic data were collected at six median openings in the Tampa Bay area of Florida. The Highway Capacity Manual (HCM) capacity estimation method was tested against capacity measured in the field using Kyte's method. It was found that the HCM method provides reasonable capacity estimates for U-turn movements at unsignalized intersections. It was also found that the HCM shared-lane capacity model overestimates the capacity of the exclusive left-turn lane, which major street left-turning vehicles share with U-turning vehicles. A linear regression model was developed to estimate the capacity of the shared left-turn lane and to quantify the effects of U-turning vehicles on the capacity of an exclusive left-turn lane. It was found that the capacity of the exclusive left-turn lane decreases with an increase in the major street traffic volume, the major street right-turn volume, and the percentage of U-turning vehicles at the exclusive left-turn lane. Assuming that the major street right-turn traffic volume is 85 vehicles per hour (veh/h) and the conflicting major street through traffic volume is 1,000 veh/h, increasing the percentage of U-turning vehicles from 10% to 50% will reduce the capacity of the exclusive left-turn lane from 290 veh/h to 250 veh/h, which accounts for about a 14% capacity reduction.