Influence of signal countdown timer on efficiency and safety at signalized intersections

Biswas, Sabyasachi; Ghosh, Indrajit; Chandra, Satish · 2017 · OpenAlex-citations

DOI: 10.1139/cjce-2016-0267

archive: archived pipeline: cataloged verified

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Summary

This study investigates the impact of Signal Countdown Timers (SCTs) on intersection efficiency and safety, addressing inconsistent findings from previous research in developing countries. The authors conducted a “before and after” study at three signalized intersections in New Delhi, India, to evaluate how SCTs influence traffic characteristics under highly heterogeneous conditions. The research specifically examines two components of SCTs: the Green Signal Countdown Timer (GSCT) and the Red Signal Countdown Timer (RSCT). The study aims to determine effects on saturation flow, start-up lost time (SULT), red light violations (RLVs), and dilemma zones (DZ). The methodology involved collecting videographic data at three intersections with similar geometric characteristics during weekdays. Data were gathered for both timer-on and timer-off scenarios, with the latter facilitated by switching off the timers via the Delhi Traffic Police. Vehicles were classified into five categories, and Passenger Car Unit (PCU) values were estimated using the headway method. The authors proposed a novel method to estimate SULT suitable for heterogeneous traffic, calculating it based on the difference between initial discharge rates and mean saturation flow rates over 5-second intervals. Safety metrics included counting RLVs during the initial and final 10 seconds of the red phase and analyzing dilemma zone ranges. Results indicated that GSCT had no significant influence on saturation flow, contradicting some prior studies but aligning with others. However, RSCT significantly reduced SULT; for instance, SULT increased from 1.64 s to 3.16 s at one intersection when the timer was off, as drivers used timer information to prepare for acceleration earlier. Regarding safety, GSCT reduced RLVs during the initial 10 seconds of the red phase by allowing drivers to adjust speeds and decide whether to stop or proceed. Conversely, RSCT increased RLVs during the final 10 seconds of the red phase, as drivers used the countdown to make early start decisions, often crossing the stop line before the green phase officially began. The timer-off scenario also enlarged the total ranges of both Type I and Type II dilemma zones. The study concludes that SCTs have distinct effects depending on their component and timing. While GSCT improves safety by reducing early red-light violations, RSCT enhances efficiency by reducing start-up lost time but may increase late-stage violations. The findings suggest that SCTs assist drivers in decision-making but require careful consideration of their mixed impacts on safety and efficiency. The proposed SULT estimation method provides a tool for analyzing intersections in heterogeneous traffic environments, contributing to better traffic engineering practices in developing nations.

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