A National Investigation on the Impacts of Lane Width on Traffic Safety

This project is one of the first and the most comprehensive efforts to date to address a long overdue built environmental challenge to health: the lack of conclusive quantitative evidence on the effects of lane width on safety which has led to unnecessarily wide travel lanes that are designed to accommodate fast and convenient driving.

This national study investigates the feasibility of narrowing vehicle lanes as the easiest and most cost-effective way to accommodate better sidewalk and bike lane facilities within the existing roadway infrastructure. The study asks whether, and to what extent, we can narrow existing vehicle lanes (for different road classifications) without adversely impacting traffic safety.

This study employed a sample of 1,117 street sections (a series of homogeneous road segments) from seven different cities and conducted one of the most comprehensive data collections on geometric and street design characteristics of street sections including bike lane type and width, median type and width, sidewalk type and width, street’s sense of visual motion, on-street parking type, width and occupancy rates, number of lanes and number of bus stops, street trees, and the degree of street curvature.

We conducted a series of four negative binomial regression analyses to investigate the relationship between lane width and the number of non-intersection crashes, after controlling for the aforementioned confounding factors. This study, to our knowledge, is the largest and most comprehensive study focusing on the impacts of travel lane width on traffic safety outcomes such as the number of vehicle accidents.

Overall, this study found no evidence that narrower lanes are associated with the higher number of crashes and that narrow lanes (9-foot and 10-foot) increase the risk of vehicle accidents, after controlling for cross-sectional street design characteristics and other confounding variables. Quite contrary, our models confirm that in some cases (in the speed class of 30–35 mph), narrowing travel lanes is associated with significantly lower numbers of non-intersection traffic crashes and could actually contribute to improvement in safety. These findings are novel with groundbreaking and immediate policy/practical implications for identifying streets in each road class as the best candidates for lane width reduction projects.

Our in-depth interviews with state DOT officials in five states also offer valuable insights on the challenges of executing lane width reduction projects and revising existing guidelines to promote narrower lanes. We also offer a range of innovative solutions that have been adopted by these states to overcome this challenge and best practices that could be applicable to other state and local departments of transportation in the country. Practical implications and policy recommendations of these findings are further explained in the report.

Download a fully copy of the report attached to this post!