Three- or Four-Section Displays for Permissive Left Turns?

Hurwitz, David S.; Monsere, Christopher M.; Marnell, Patrick; Paulsen, Kirk · 2014 · Crossref

DOI: 10.3141/2463-01

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Summary

This study investigates whether the vertical configuration of traffic signal heads—specifically three-section versus four-section displays—affects driver performance during permissive left turns controlled by a flashing yellow arrow (FYA). While the Manual on Uniform Traffic Control Devices (MUTCD) mandates four-section heads for protected-permissive operations, some jurisdictions utilize three-section heads for cost or space reasons, displaying the FYA in the same lens as the protected green arrow. The research addresses a gap in existing literature, as prior studies had not examined how the vertical positioning of the FYA influences driver comprehension or safety, despite concerns regarding visual search tasks and potential confusion for drivers with certain forms of color blindness. The researchers conducted a simulator-based experiment using a high-fidelity, motion-based driving simulator equipped with mobile eye-tracking technology. Twenty-seven participants completed a course involving 24 simulated intersections, resulting in 620 analyzable permissive left-turn maneuvers. The experimental design controlled for opposing traffic volume, pedestrian presence and walking direction, and signal head configuration. Driver performance was assessed through two primary metrics: average total fixation duration (ATFD) on specific areas of interest (AOIs), such as the signal head, opposing vehicles, and pedestrians; and the position of pedestrians in the crosswalk at the moment the driver initiated the turn. Statistical analysis employed Welch’s t-tests to compare performance between the three-section and four-section configurations. The results indicated no statistically significant differences in average fixation durations between the two signal configurations for any measured AOI, including the signal head itself, opposing vehicles, or pedestrian areas. Drivers spent the most time fixating on opposing vehicles regardless of signal type. Regarding pedestrian safety, the position of pedestrians when drivers initiated turns was not significantly different for three of the four pedestrian scenarios. However, a statistically significant difference was found when a single pedestrian was walking away from the driver: drivers using the four-section signal waited until the pedestrian was closer to the destination curb (average position 0.72) compared to those using the three-section signal (average position 1.08). This suggests drivers with the four-section display were slightly more cautious in this specific context. The study concludes that measurable driver performance, particularly visual attention and general turning behavior, is not sensitive to the vertical positioning of the FYA display in the permissive interval. The findings challenge the assumption that four-section signals are inherently superior for driver comprehension or safety in permissive left-turn operations. The results suggest that the MUTCD requirement for four-section heads may not be strictly necessary for performance reasons, supporting the potential use of three-section displays where cost or physical constraints are factors. However, the authors note limitations, including the simulator environment and the specific demographic of participants, suggesting further field validation is warranted.

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