Investigation of Median Trees and Collisions on Urban and Suburban Conventional Highways in California

This study examines safety in the presence of large trees in curbed medians of conventional highways that are also principal streets in developed urban and suburban areas. Statistical modeling methods were used to associate collision frequency and severity with highway and traffic characteristics, with and without median trees. The methods include simple accident rates and three types of multivariate modeling and use collision data for 6 years. The analysis was done in two parts. The first part was a screening investigation of pertinent facilities under both state and local jurisdiction. The second part, described here in detail, focused on 14,283 collisions occurring on 58 mi (99 km) of designated state highways in 29 different sections, 19 with median trees. The analysis examined different subsets of reported collisions, in turn omitting collisions on the right side of the road, collisions not having median or median-shoulder involvement, and collisions at intersections. The overall conclusion is that large trees in medians of major conventional urban and suburban highways of state jurisdiction are associated with more collisions and increased severity. However, some of these associations are statistically weak. For the situations examined, lower speeds and larger side clearances were not found to mitigate the increased collision impacts associated with median trees.