Exploratory Analysis of Relationship Between Number of Lanes and Safety on Urban Freeways

The relationship between freeway capacity and the number of lanes is reasonably well understood at present. In contrast to capacity, the relationship between the number of lanes and safety is not fully understood or systematically considered when planning capacity improvements. During the planning and design phase the discussion is centered on the degree to which design alternatives comply with geometric design standards and what Level of Service (LOS) is provided. It is generally believed by the practitioners that additional capacity afforded by additional lanes is associated with more safety. How much safety and for what time period is generally not considered. Exploratory analysis of the Safety Performance Functions (SPF) for multilane freeways in Colorado, California and Texas suggests that adding lanes may initially result in a temporary safety improvement that disappears as congestion increases. As Annual Average Daily Traffic (AADT) increases, the slope of SPF, described by its first derivative, becomes steeper, reflecting that accidents are increasing at a faster rate than would be expected from a freeway with fewer lanes. This may possibly be explained by the fact that increase in the number of lanes is associated with increase in the number of potential lane-change-related conflict opportunities. Understanding of the relationship between number of lanes and safety should be used to inform the public involvement process in evaluating and selecting design alternatives. Additionally, high AADT on multilane freeways is associated with high crash frequency and consequently reduced mobility.