A Study of Driver Behaviors at Unsignalized Intersections Using SHRP2 Naturalistic Driving Study Data and Field Observed Conflict Data

Abstract

Unsignalized intersections on divided highways with wide medians (>30 ft) have the potential for severe crashes due to numerous conflict points and high speed. Drivers making minor road left turns have to make multiple judgments when crossing the intersection, exposing them to a higher risk environment. Therefore, the purpose of the study is to analyze the minor road left turn driver behaviors (e.g., driver visual workload, speed change behavior, stop and conflict behaviors) at unsignalized intersections with wide medians on high-speed divided highways in rural or suburban areas. The study also made suggestions on improving the intersection safety design based on the driver behaviors. collected a total of 440 left turn trips at six conventional unsignalized intersections, and 40 right-turn followed by U-turn trips at Restricted Crossing U-Turn (RCUT) intersections from the Second Strategic Highway Research Program (SHRP 2) Naturalistic Driving Study (NDS) database. Driver eye-glance data, speed change, brake pedal usage, stop condition and the roadway feature data were analyzed. Additionally, the study conducted conflict study between six pairs of unsignalized intersections in the state of Alabama to assess the safety effect of the two types of median opening access control treatments: (1) stop bars, stop signs, and double yellow line and (2) yield lines, yield signs and double yellow line.

The study first analyzed the NDS data of driver eye-glance, demographic information, and roadway features. The entropy rate of each trip was calculated as an indicator of the driver visual workload and was treated as the dependent variable for the statistical analysis. The higher the entropy rate the higher the workload. A comparative study between these two movements at these two types of intersections was conducted. Results indicated that drivers at RCUT intersections have less random scanning and longer average fixation and spent more than 70% of time looking forward during the whole movement. Younger drivers at both types of intersections have higher entropy rates. Additionally, drivers at conventional intersections with higher AADT (≥ 30,000) have higher entropy rates. The analysis of the driver speed change and stop behaviors were conducted for three different phases, including phase 1 - Deceleration, phase 2 - Intersection Entry, and phase 3 - Execute Turn. Study results show that 85th percentile of the left turn drivers tend to decelerate sharply when they are about 50 feet away from the minor road stop line; about half of the drivers did not make complete stops at the minor road; only 25% of the drivers stopped at the median opening; and 85th percentile of drivers used up to 650 ft to speed up to 45mph. Additionally, a cross-sectional comparison was conducted between six pairs of unsignalized intersections (with access controls vs. without access controls at the median openings) in the state of Alabama. Video cameras were installed to monitor each intersection during a typical weekday. Two specific movements: minor road left-turn movement and major-road left-turn movement are significantly affected by the two median opening treatments. The study includes analyzing 16 hours of video for each location to record the number of traffic conflicts, near-crash situations, and left-turn driver behaviors (defined as understanding right-of-way, whether or not stopping at the median opening, and using two-stage left-turn movements). The results showed that the stop bars at the median opening can reduce the traffic conflict rate by 10% to 40% and help more drivers to stop or slow down at the wide median openings to make a better judgment of the safe gap. The stop and yield lines combined with a double yellow line can also help the left-turn drivers better understand the right-of-way and reduce the right-angle conflicts.

The study results will provide a better understanding of driver behaviors and the behaviors’ safety implications at different types of unsignalized intersections. The corresponding suggestions for improving the roadway designs were also summarized at the end of the study.