Safety By Design: Quantifying the Impact of Urban Design on Traffic Collision, Injury, and Fatality Rates

Mariela Alfonzo (State of Place/New York University)

Every 25 seconds, someone dies in a traffic collision. In response, over 35 U.S. cities have created Vision Zero programs (or the equivalent) to make streets safer. But most cities lack data to identify effective ways to eliminate road injuries and fatalities. Additionally, given limited capacity, cities prioritize certain interventions over others, often based on anecdotal evidence, best practices, or simply intuition and gut. Accordingly, many cities struggle to justify “costly” urban design interventions, opting for more affordable enforcement and education strategies that may not be as effective as built environment changes. This study aimed to understand the relationship between urban design and traffic collisions and motorist, pedestrian, and bicyclists injuries and deaths. We compared the “State of Place Index” (a score from 0-100 that aggregates data on 290 urban design features at the block level) of a stratified random sample of 60intersections in Durham, North Carolina and 302 adjacent blocks to 362 “control” blocks randomly sampled from an existing database of 6892 blocks. Based on preliminary results of a binomial logistic regression, we found that for every 20 points decrease on the State of Place Index, a collision, injury, or fatality was 20 times more likely to occur. We also found that urban form, density, proximity to non-residential destinations, parks and public spaces, pedestrian and bicyclist amenities, personal safety, traffic safety, aesthetics, and connectivity were significantly related to traffic incidents. These results have implications for traffic engineers, planners, and urban designers, signalling that urban design strategies must be integrated within current efforts to create safer streets for motorists, pedestrians, and bicyclists. Additionally, as the State of Place Index is also tied to higher return on investments, based on increased real estate premiums, citymakers can use this methodology to economically justify improvements to urban design and safety.