Design of Traffic Signal Clearance Intervals to Consider Impacts of Buses

Abstract

Traffic signal design procedures use only light-duty vehicles in the design of yellow and clearance intervals. For quantification of the impact of buses on yellow and clearance times, a controlled field experiment was conducted. In the study, 864 stop and run records were collected for a 56 km/h (35 mph) approach speed; participant bus drivers encountered a yellow indication initiation at different distances from the intersection. Participant drivers were selected randomly from three age groups (i.e., younger than 40 years old, 40 to 64 years old, and 65 years or older), and an equal number of male and female participants was included in each age group. Data collected were used to generate two linear mixed models: one for bus driver perception–reaction times and one for deceleration levels. Both models considered roadway surface and environmental parameters, driver attributes (i.e., age and gender), roadway grade, approaching speed, and time and distance to the intersection at the onset of yellow. The proposed models were used to perform a Monte Carlo simulation to generate yellow timing and all-red durations. Lookup tables were generated for the proposed yellow timing duration for different approach speeds, roadway grades, confidence intervals, precipitation, and percentage of buses in the traffic stream to provide practical guidelines for the design of traffic signal clearance times.