Designing Traffic Signal Yellow and Change Intervals considering Truck Impacts

This research quantifies the effect of heavy vehicles on the design of yellow and all-red intervals. With empirical data from previous studies and truck performance characteristics, models are developed to simulate driver perception–reaction times and truck deceleration levels at signalized intersections. These models consider roadway surface and weather conditions (dry, fog, or rain); driver attributes (age and gender); roadway grade; approach speed; and time and distance to the intersection at the onset of the yellow indication. An agent-based approach is used to develop driver- and vehicle-specific yellow times. The yellow timings for light-duty and heavy-duty vehicles are then combined and consider different percentages of heavy vehicles in the traffic stream. Lookup tables are developed for grades, precipitation types and levels, and percentages of trucks to provide practical guidelines for the design of yellow and all-red signal timings. In general, the research demonstrates that adding trucks to the traffic stream increases yellow times by 2% to 20%, depending on approach speed, grade, and weather conditions, with the most significant increases occurring on dry roadways with high positive gradients at lower speeds. In absolute terms, the yellow time is increased from approximately 0.1 to 0.6 s when trucks constitute 5% of the traffic stream to 0.6 to 1.4 s when trucks are 30% of the traffic stream. The study also demonstrates a need to increase the all-red time by up to 0.5 s when trucks comprise 30% of the traffic stream.