Models of vehicle operating speeds along two-lane rural highway transition zones: Panel and multilevel modeling approaches

Transition zones are defined as locations where the posted speed limit changes from high- to low-speed. On high-speed two-lane rural highways, transition zones are often encountered when the roadway approaches a built-up area. The purpose of this study was to collect operating speed, roadway, roadside, traffic control, and land use data along two-lane rural highway transition zones in central Pennsylvania. These data were used to estimate passenger car operating speed models, recognizing the hierarchical nature of the data-generating process. The results showed that a three-level model could capture site-level speed variance that could not be captured in a panel data modeling framework; however, the parameter estimates and standard errors were very similar when comparing the model estimation results. It was also found that the presence of horizontal curves, presence of warning signs, the presence of curb, and increased access density were associated with reduced vehicle operating speeds. Increasing the lane width, shoulder width, and lateral clearance to obstructions was associated with increases in vehicle operating speeds. The models estimated in this paper could be used as a starting point to develop transition zone design guidelines.