A Field and Simulator Evaluation of All-Red Clearance Intervals for Use in Left-Turn Applications

Knodler, Michael A.; Santiago-Chaparro, Kelvin R.; Christofa, Eleni; Bill, Andrea R.; Tainter, Francis; Noyce, David A. (David Alan), 1961- · 2018 · ROSA P / Safety Research Using Simulation (SAFER-SIM) University Transportation Center

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Summary

This study addresses the lack of national guidance regarding change and clearance intervals during the transition from protected to permissive left-turn phases, specifically when using the flashing yellow arrow (FYA). While the FYA is recognized as the most effective indication for permissive left turns, the specific signal sequencing—particularly the duration of solid yellow and all-red intervals preceding the FYA—varies across jurisdictions. The research aims to develop recommendations for these intervals by evaluating driver comprehension and real-world behavior, hypothesizing that standardized guidelines will improve safety and operational efficiency. The methodology employed a two-pronged approach: a computer-based static evaluation and a field study utilizing vehicle trajectory data. The static evaluation surveyed drivers using randomized video sequences of protected-permissive left-turn (PPLT) phasing with both circular green (CG) and FYA indications. Participants were asked to predict the next signal phase after viewing specific sequences, covering various phasing schemes such as dual leading and lead-lag. The field study collected high-resolution vehicle trajectory data at intersections in Amherst, MA, and Appleton, WI, using the Intersector™ radar-based detection system. This system logged vehicle position, speed, and timestamps at 2 Hz, which were merged with signal status data recorded via video to analyze driver acceleration and deceleration profiles relative to signal changes. Results from the static evaluation revealed differences in driver comprehension between CG and FYA indications across different phasing schemes. The analysis of correct responses indicated that driver understanding varied depending on the specific sequence and indication type. Preliminary findings from the field trajectory data showed that drivers frequently utilized the all-red clearance interval as an extension of the protected phase rather than a distinct clearance period. This behavior was observed in both CG and FYA scenarios, suggesting that drivers often treat the all-red interval as additional protected time, potentially impacting the intended safety function of the clearance interval. The trajectory data provided detailed insights into how vehicles decelerated or maintained speed during these transitions, highlighting discrepancies between expected and actual driver behavior. The significance of this research lies in its contribution to the development of standardized guidelines for PPLT signal sequencing. By combining static comprehension data with empirical field observations, the study provides a foundation for future driving simulator evaluations and policy recommendations. The findings underscore the need for clear national standards on change and clearance intervals to mitigate red-light running and improve intersection safety. The use of radar-based trajectory data demonstrates a novel method for analyzing driver behavior in real-world conditions, offering a robust tool for future traffic engineering research. Ultimately, the study supports the refinement of FYA implementation to enhance both safety and operational efficiency at signalized intersections.

Key finding

Driver comprehension of protected-permissive left-turn signal sequences varied significantly across different phasing schemes, and radar-based trajectory data successfully captured vehicle speed profiles during these transitions.

Methodology

mixed_methods

Provenance

The full processing record for this entry. Every stage of this paper's journey through the pipeline is logged — what ran, with which tool and model, how many attempts it took, and when it last completed. Discovered via bulk_ingest_rosap on 2026-05-23 (6 acquisition events logged).

Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

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• signaling behavior