The Unresolved Relationship between Street Trees and Road Safety

Streets – especially in urban areas – serve as far more than simple conduits of vehicular traffic. They are a place for pedestrians and bicyclists; they are a place for children to play and learn; and they are part of the community as a livable and healthy environment. Trees can play a significant role in these functions. In his research on livable streets, Donald Appleyard discusses how trees “provide relief from the hardness and grayness of the city” and help “provide shade in the summer and remind people of the natural environment, which is often far away; they signal the seasons, and symbolize, through growth, flowering and decay, the cycles of life itself” (Appleyard 1980). The research points to street trees being associated with a better pedestrian environment, reduced urban head island effects, less drainage infrastructure as well as a host of other social, environmental, and even economic benefits (Burden 2006, Ewing and Dumbaugh 2009). Dan Burden points out that planting a tree, including three years of maintenance, costs between $250 and $600, but that the economic return of a single street tree seems to far exceed the initial costs (Burden 2006). By estimating the value of air conditioning savings, erosion control, wildlife habitat, and air pollution reduction over the life of an average tree, the American Forestry Institute estimated this benefit as $57,151 in 1992 (or $96,196 in 2014 dollars) (Moll and Young 1992). This estimate does not even account for the research that suggests that street trees add value to adjacent homes, businesses, and the tax base (Das 1979, Burden 2006). Considering the extensive list of benefits, it is no wonder that street trees have long been a staple of good urban design and shaping more livable spaces. In fact, they seem to be beneficial in most every way except one: road safety. The national safety data suggests that car/tree collisions account for more than 4,000 fatalities and 100,000 injuries in the U.S. each year (FHWA 2006). So while traffic engineers acknowledge that trees can be an asset, they also point out that trees are the “single most commonly struck objects in serious roadside crashes” (FHWA 2006). To remedy this issue, traffic engineers prefer that trees and other “fixed- object hazards” be located a safe distance from the roadside. This area where fixed- object hazards are minimized is called the clear zone, which has been standard design practice since the 1967 American Association of State Highway Officials (AASHO) publication of Highway Design and Operational Practices Related to Highway Safety, which cited the need for a 6- meter (19.7’) clear zone without any trees larger than 4” caliper (AASHO 1967). Soon thereafter, the recommended lateral clearance increased to 9-meters (29.7’) and explicitly included both rural and urban locations. While today’s traffic engineers acknowledge that trees on low-speed residential streets “do not usually present the same problems as trees near high-speed roads and highways” and recognize that urban right-of-ways are often extremely restricted, the 2011 American Association of State Highway and Transportation Officials (AASHTO) Roadside Design Guide continues to encourage clear zone application wherever practical. Despite standard design practice, the research remains conflicted over the true association between street trees and road safety (Zeigler 1986, Turner and Mansfield 1989, Dumbaugh 2005, Gattis 2005, Ivan et al. 1999, Naderi, Kweon, and Maghelal 2008, Ossenbruggen, Pendharkar, and Ivan 2001, Wolf and Bratton 2006). This proposed research seeks to better understand this issue by: i) developing a methodology for deriving tree canopy data via remotely sensed data; ii) collecting an extensive database of road crashes, traffic counts, and other relevant crash factors; and iii) conducting a statistical analysis of roadside trees and road safety. The goal is to shed light on the true relationship between street trees and road safety outcomes.


  • English


  • Status: Completed
  • Funding: $270746
  • Contract Numbers:


  • Sponsor Organizations:

    Research and Innovative Technology Administration

    University Transportation Centers Program
    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590
  • Managing Organizations:

    Mountain-Plains Consortium

    North Dakota State University
    P.O. Box 6050, Department 2880
    Fargo, ND  United States  58108-6050
  • Project Managers:

    Kline, Robin

  • Performing Organizations:

    Dept. of Civil Engineering

    University of Colorado Denver
    Denver, CO  United States 
  • Principal Investigators:

    Marshall, Wesley

    Janson, Bruce

  • Start Date: 20150805
  • Expected Completion Date: 20190731
  • Actual Completion Date: 20190325
  • USDOT Program: University Transportation Centers Program
  • Source Data: MPC-489

Subject/Index Terms

Filing Info

  • Accession Number: 01579924
  • Record Type: Research project
  • Source Agency: Mountain-Plains Consortium
  • Contract Numbers: DTRT13-G-UTC38
  • Files: UTC, RiP
  • Created Date: Oct 27 2015 4:00PM