Urban clear zones, street trees, and road safety

The roadside area where fixed-object hazards are explicitly minimized is called the clear zone, which became standard design practice soon after the 1966 Congressional hearings on road and automobile safety. Mounting evidence, however, is beginning to cast doubt on what we think we know about the impact of roadside clear zones on actual safety outcomes. This is particularly an issue with street trees in urban contexts, which are known to provide economic, environmental, and livability benefits but are also widely considered a road safety detriment.This research relies upon advances in remote sensing to map both tree canopy and street-tree locations in GIS for the entirety of the city and county of Denver, Colorado. The authors then statistically test the association between street trees and seven years of road safety outcomes while controlling for factors known to be associated with crash outcomes.Despite fifty years as standard design practice, the results suggest that the expected safety benefit of roadside clear zones – at least with respect to street trees in an urban context – may be overstated. In fact, larger tree canopies that extend over the street are associated with fewer injury/fatal crashes as well as fewer crashes overall while holding all other variables constant. The number of street trees per mile associates with improved safety in wealthier neighborhoods but can be detrimental in low-income neighborhoods; this inconsistency represents an equity issue in need of future research. When assessing the safety impact of street trees in the clear zone, municipalities and transportation agencies need to be more cognizant of how street design may impact road user behaviors, particularly related to issues that directly affect safety such as travel speeds and driver awareness.


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  • Accession Number: 01683101
  • Record Type: Publication
  • Files: TRIS
  • Created Date: Sep 28 2018 3:04PM