Assessing the Impact of Roadway and Roadside Features on Safety in Urban Highways: A Case Study in Red Wing City, Minnesota

Roadway and roadside design features play a critical role in influencing drivers’ behavior and safety outcomes. Nonetheless, some roadway and roadside design features show opposing influences on driver’s speed choice and crash risk in different land use contexts (e.g., urban, suburban, rural). This study quantifies how urban roadway and roadside design features affect crash frequency and operating speed, using features extracted from Light Detection and Ranging (LiDAR) data in Red Wing, Minnesota, United States. The results showed that the presence of medians, lower posted speed limits, and higher roadway operating speed was the strongest determinant of crash risk (associated with lower crash frequency for medians and operating speeds, and with higher run-off-road [ROR] crash frequency for higher speed limits). On undivided segments, denser tree clusters substantially increased crash risk: increasing tree density from 50 to 70 clusters per mile was associated with a 28% increase in total crashes and a 45% increase in ROR crashes. On the other hand, when a median (raised/depressed) was present, this adverse tree-density effect was largely offset, indicating that geometry and roadside vegetation interact rather than act independently. Greater roadside tree lateral offset reduced ROR crashes. Operating speed was primarily governed by traffic volume and segment geometry (e.g., posted speed limits, parking/curb presence, and sidewalk/shoulder configuration), with roadside vegetation contributing small effects (about a 2% reduction in operating speed per 10% increase in canopy coverage). Findings of this study contribute to existing roadway and roadside design guidance by showing that geometric consistency, median treatments, and strategic tree placement with adequate clearance can be used in constrained urban contexts to improve safety, while greater canopy coverage provides a low-speed calming effect.