Impact of using indirect left-turns on signalized intersections’ performance

Taha, Mahmoud; Abdelfatah, Akmal · 2017 · Crossref

DOI: 10.1139/cjce-2016-0259

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Summary

This study investigates the operational performance of unconventional left-turn treatments at signalized intersections, specifically comparing direct left-turns (DLT) with indirect methods: right-turn followed by U-turn (RTUT) and U-turn followed by right-turn (UTRT). Motivated by limited right-of-way and the need to reduce traffic delays without extensive geometric changes, the research aims to determine which control type performs best under varying traffic conditions and U-turn locations. The primary objective is to provide recommendations for optimizing signal efficiency and reducing overall delay by eliminating direct left-turn conflicts. The methodology employed microscopic simulation using VISSIM software to model three intersection configurations. Signal timing was optimized using Synchro software before being applied in the simulations. The study analyzed 420 scenarios, varying parameters including total traffic volume (low to very high), traffic distribution across approaches, left-turn percentages (15%, 30%, and 45%), and U-turn location distances from the main intersection (50, 100, and 200 meters). Performance was evaluated based on overall delay, travel time, vehicle kilometers traveled (VKT), and Level of Service (LOS). Results indicate that unconventional left-turn treatments generally yield lower delay and travel time compared to DLT when U-turn locations are situated 200 meters from the intersection. At this distance, UTRT demonstrated the least delay across all traffic volumes because vehicles stop only once, whereas RTUT vehicles may stop twice. However, when U-turns are located 100 meters away, RTUT showed superior performance over other types. Conversely, placing U-turns too close (50 meters) caused significant performance degradation due to queue spillback and weaving conflicts, particularly at high volumes exceeding 6,000 vehicles per hour. In these close-proximity scenarios, DLT often outperformed unconventional treatments. While indirect turns increased VKT by less than 8%, they improved intersection capacity by reducing signal phases from four to two. The study concludes that the effectiveness of indirect left-turns is highly dependent on the separation distance between the intersection and the U-turn location. RTUT is recommended when U-turns are approximately 100 meters away, while UTRT is preferable at 200 meters. Implementing these treatments can significantly reduce delays and improve Level of Service under moderate to high traffic volumes, provided the U-turn location is optimized to avoid spillback effects. This research offers practical guidance for transportation agencies seeking to enhance intersection efficiency through unconventional control strategies.

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• intersection design