Evaluating urban intersections: a comparative study of roundabouts versus protected U-turns—which is better?

Alnawmasi, Nawaf; Jashami, Hisham; Cai, Qiuqi · 2025 · Crossref

DOI: 10.55329/jbwz1736

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Summary

This study addresses the challenge of managing traffic congestion and safety in rapidly urbanizing areas by comparing the operational performance of traditional roundabouts against protected U-turns. While roundabouts are known for reducing severe crashes, they often suffer from inefficiency and increased congestion under high-traffic volumes. Protected U-turns, a modern geometric design intended to handle high volumes more effectively, were evaluated as a potential alternative. The research specifically examines these intersection types within the context of Ha’il City, Saudi Arabia, aiming to determine which design offers superior traffic flow and capacity management. The methodology employed PTV VISSIM 2023, a microscopic traffic simulation software, to model and analyze traffic dynamics. The study focused on a specific three-lane roundabout located on King Saud Road, which experiences high traffic volumes. Traffic data collected over five days indicated that 90% of vehicles traveled in the eastbound/westbound directions, with an average daily traffic of 76,465 vehicles for those directions and 5,047 for northbound/southbound. The simulation inputs reflected this distribution, using 4,500 vehicles per hour for major roads and 500 for minor roads. Two protected U-turn models were developed based on the existing geometry: one with a three-lane road and two-lane U-turn, and another with a two-lane road and one-lane U-turn. Performance was evaluated using measures of effectiveness including average vehicle delay, level of service (LOS), and queue length. The results demonstrated a significant disparity in performance between the two designs under high-volume conditions. The three-lane roundabout operated at LOS F, the worst traffic condition, with an average vehicle delay of 84.19 seconds and an average queue length of 385.31 feet. In contrast, both protected U-turn models operated at LOS A, the best condition. The three-lane road with a two-lane U-turn recorded an average delay of 0 seconds and a queue length of 6.83 feet, while the two-lane road with a one-lane U-turn showed a delay of 0.04 seconds and a queue length of 14.41 feet. Sensitivity analysis revealed that the roundabout’s performance degraded sharply as volume increased, reaching LOS F at 1,600 vehicles per hour, whereas protected U-turns maintained low delays and queue lengths even at the highest simulated volumes of 4,500 vehicles per hour. The study concludes that protected U-turns are significantly more effective than roundabouts for managing high-traffic volumes in urban settings, offering superior intersection throughput and reduced congestion. The findings suggest that city planners and traffic engineers should consider protected U-turns as a viable alternative to traditional roundabouts in rapidly growing cities. However, the authors note limitations, including the lack of safety analysis regarding crash rates and environmental impact assessments such as emissions. Future research is recommended to explore these safety and sustainability factors, as well as the applicability of these findings to different urban contexts and roundabout configurations.

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• intersection design