Citation

Adams JC, Hummer JE. Transp. Res. Rec. 1993; 1398: 90-100.

Copyright

(Copyright © 1993, Transportation Research Board, National Research Council, National Academy of Sciences USA, Publisher SAGE Publishing)

DOI

unavailable

PMID

unavailable

Abstract

As more U-turning vehicles use a left-turn lane, the saturation flow rate of the lane may become significantly lower. However, the 1985 Highway Capacity Manual (HCM) does not account for U-turns in calculating the capacity of a left-turn lane group at a signalized intersection. To determine whether a U-turn factor should be included in a revised HCM analysis method, a preliminary study was conducted at North Carolina State University. The study team selected four intersections with exclusive left-turn lanes and protected signal phasing and recorded saturation flow rates and U-turn percentages for 198 queues during weekday midday peaks. The data analysis showed that a saturation flow reduction factor appears necessary for left-turn lanes that have large percentages of U-turns. T-tests and regression models indicated that saturation flow rates were significantly lower when queues had more than 65% U-turns. However, the analyses also showed no correlation between saturation flow and the percentage of U-turns for queues with 50% or fewer U-turns. The analysis was inconclusive between 50 and 65% U-turns because of small samples. The results suggest tentative saturation flow reduction factors of 1.0 for U-turn percentages below 65, 0.90 for U-turn percentages between 65 and 85, and 0.80 for U-turn percentages exceeding 85. A follow-up investigation should focus on intersections that have high percentages of U-turns, restrictive geometry, or high percentages of U-turning heavy vehicles.

Record URL:
http://onlinepubs.trb.org/Onlinepubs/trr/1993/1398/1398-013.pdf

Language: en

Keywords

Mathematical models; Traffic control; Traffic signals; Motor transportation; Highway systems