July 20, 2009

Traffic Safety and Community Design:
What Is the Relationship?

A JAPA article looks at how the design of our communities may lead to traffic crashes, injuries, and deaths.

Arterial roadways, strip commercial uses, and big box stores all have a profoundly negative effect on traffic safety, while the presence of traditional, "main-street"-type retail configurations are associated with safer communities, according to an article in the Summer 2009 issue of the Journal of the American Planning Association.

The article, "Safe Urban Form: Revisiting the Relationship Between Community Design and Traffic Safety," analyses three years of traffic accident data for the City of San Antonio to understand how different land use and street configurations may influence crash incidence.

One of its most important conclusions is that many current assumptions about how community design practice affects traffic safety are not supported by empirical evidence. The article reviews the connection between traffic safety and community design practices since the beginning of the 20th century, including:

  • Arterial thoroughfares
  • Routing traffic away from residential zones
  • Reducing numbers of four-way intersections
  • Disconnected residential subdivisions and cul-de-sacs
  • Retail and commercial uses located on arterial roads designed for heavy traffic volumes

The study matched data on crash incidence at the block group level in the City of San Antonio (a city of 1.4 million residents) with data on land use, street network characteristics, traffic volumes, and community demographics, and then analysed them using a series of negative binomial regression models to predict the incidence of total, injurious, and fatal crashes.

Results include finding a 6.6 percent increase in total crash incidence for each big box store located adjacent to an arterial thoroughfare and a 2.2 percent decrease in crash incidence for each pedestrian-scaled commercial or retail use. Population density increases were associated with fewer crashes. Each additional mile of arterial roadway within a block group was associated with a 15 percent  higher incidence of crashes.

The authors note that the speed of modern vehicles makes urban arterials particularly problematic. Though they are designed to address regional mobility functions, they are also commonly shopping corridors. When asked about these findings, Dumbaugh replied, "from a traffic safety perspective, the modern commercial arterial is a perfect storm of bad planning and design. These roads are designed to support high operating speeds, making it difficult for drivers to stop quickly to avoid a crash, and the presence of commercial and retail uses on these roads means that drivers will routinely need to stop quickly in order to avoid crashing into pedestrians, bicyclists, and especially vehicles turning in and out of driveways."

While the presence of big box stores and strip commercial uses led to significant increases in crash incidence, the presence of more urban, pedestrian-oriented retail configurations was associated with a significant reduction in traffic crashes, a finding which helps illustrate the role of design. "We're seeing this in a growing number of studies, both here in the U.S. and in Europe" Dumbaugh said. "People adapt their driving behavior to the design of the built environment. Higher-density, more traditionally-designed retail streets tell drivers that they should expect crossing vehicles and pedestrians, and they respond by reducing their speeds and being more attentive."  

Based upon the data obtained and the results, the authors make three suggestions for future design enhancements to roads and communities: manage mobility and access functions of urban arterials; relocate retail and commercial uses to lower-speed thoroughfares (or reduce speeds on roads already lined with commercial uses); and give greater attention to how land use planning may affect crash incidence. The article concludes that the relationship between community design and traffic safety are very important, and calls for a more thoughtful, evidence-based approach to addressing traffic safety through community design.

About the Authors
The authors are Eric Dumbaugh of Texas A&M University and Robert Rae of Kimley-Horn and Associates. Dumbaugh is an assistant professor at the University and is the author of 25 articles on traffic and safety issues such as "Safe Streets, Liveable Streets" (JAPA, 2005) and "The Design of Safe Urban Roadsides" (Transportation Research Record: Journal of the Transportation Research Board, 2006). Robert Rae is an analyst for Kimley-Horn and studied with Dumbaugh as a master's student during the time the data for the paper was collected and analysed.

Journal of the American Planning Association (JAPA)
JAPA publishes only peer-reviewed, original research and analysis. It aspires to bring insight to planning the future. For more than 70 years, it has published research, commentaries, and book reviews useful to practicing planners, policymakers, scholars, students and citizens of urban, suburban and rural areas.

For more information about the journal and for free access to the full article, please visit:
www.informaworld.com/smpp/content~db=all~content=a911996851

Contact

Eric Dumbaugh
Assistant Professor
Department of Landscape Architecture and Urban Planning
Texas A&M University
edumbaugh@tamu.edu  
Phone: +001 (979) 862 4320