Passing Zone Behavior and Sight Distance on Rural Highways: Evaluation of Crash Risk and Safety under Different Geometric Conditions
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To determine the effect of horizontal curvature, vertical curvature, and guardrails on driver passing behavior, a combination of field data collection and a driving simulator in a controlled environment were used. Field sites were identified along areas of concern to the Alaska Department of Transportation and Public Facilities: 1) Parks Highway (Milepost 155–160), Seward Highway (Milepost 104–113), and Sterling Highway (Milepost 145.5–150.5). Data were collected in the form of video in one passing zone in each these segments (totaling 165 hours), processed, and used to inform the development of the driving simulator. Driving participant testing was conducted in a simulated environment that replicated the identified sections of the two-lane rural highway in the state of Alaska. The results of the field study showed that on all three highway segments (Parks, Seward, and Sterling), the average speed of passing vehicles was approximately 10 mph over the posted speed limit. The average initial speed of the vehicles being passed was approximately 2 mph over the speed limit. Standard deviations for both passing and impeding vehicles ranged from 3 mph to 7 mph. The field study revealed a significant number of “early start” and “late finish” passes, where a portion of the passing maneuver occurred outside of the designated passing area. Seventy-two participants were recruited and tested in a driving simulator at the Idaho Visual Performance Laboratory. The results of the simulator study showed that both horizontal and vertical curvature have significant effects on the characteristics of passing maneuvers including the speed of the passing vehicle, the total time and distance of the maneuvers, and the distance between the passing vehicle and the impeding vehicle at the initiation and termination of the maneuvers. Though geometry has an effect on passing choice and passing maneuver characteristics, it has no significant effect on safety outcomes of passes (in terms of total time to collision). Presence of guardrail does not affect the number of passes or the safety outcomes of passes but does affect the ability of drivers to avoid a collision when another vehicle is in their lane.