Examination of Roadway Characteristics Affecting Driver Speed Selection in Speed Transition Zones

Abstract

Recent approaches to speed management have shifted from focusing on the 85th percentile speed to a more comprehensive, safety-focused approach. This new approach incorporates thoughtful design, appropriate speed limits, education, and enforcement to create safer environments for all road users. This includes the incorporation of a Complete Streets approach to roadway design, which uses cross-sectional characteristics that reduce speeds and create a more accommodating environment for people biking and walking. This study examines how drivers adjust their speeds as they transition from high-speed roadways to low speeds based on contextual factors related to the roadway environment. Speed data were collected from 19 highway corridors across Minnesota using handheld lidar guns as drivers transitioned from high-speed rural highways to lower-speed rural and suburban communities. The study results in the estimation of a series of speed-reduction factors, which detail the impacts of various site-specific characteristics on travel speeds. Various features are shown to serve as effective speed-control measures, such as single-lane roundabouts, which reduced speeds by about 7 mph. Speeds were also lower on segments that included two-way, left-turn lanes (0.7 mph), depressed medians (1.2 mph), and raised medians (3.1 mph). The results also showed that drivers typically begin reducing their speeds approximately 800-ft upstream of posted speed limit signs and continue to reduce their speeds to a distance 400-ft beyond the sign location. The measures that were found to reduce speeds were also associated with lower variability in speeds.