172 Wolf and Bratton: Urban Trees and Traffic Safety Specifics of accident rates and crash circumstances in ur- ban areas are not well understood. Designers are urged to remove trees or install protective devices where strategically appropriate to reduce crash risk (such as on curves or to protect sight lines). Generally applied clear zone practices and the ab- sence of empirically based urban design standards makes such fine-grain decisions difficult for urban road designers. Roadside Tree Benefits Arborists and urban foresters are well aware of the many benefits and functions that trees provide in cities. Extensive research has documented environmental, social, and eco- nomic benefits for communities, municipalities, and regions (Nowak and Dwyer 2000; Wolf 2004). Unfortunately, this empiric evidence is not yet widely acknowledged within the transportation industry. The consequence is that tree crash statistics are often weighed against anecdotal reports of tree benefits in transportation decision-making. Discussions of roadside trees are largely framed as aesthetic values that, when compared against nominal safety standards, may not be viewed as adequate justification for tree retention or planting. Earlier transportation publications promoted trees. Neale (1949) proposed that “trees have undoubtedly saved many lives and prevented many accidents in intangible ways.” He observed that well-spaced trees might improve driver comfort by providing relief from the sun and wind. Trees can help prevent snow drifting, keep drivers alert, and add beauty to harsh roadways. Trees can reduce storm water runoff and soil erosion as well as keeping dust levels low on roadways. Trees in medians can cut cross-glare. Zeigler (1986) also observed benefits: shade, windbreaks, visual buffer, physical protec- tion for pedestrians from run-off-the-road vehicles, and con- tributions to historic character. Investigators have begun to systematically examine the many benefits of the roadside urban forest. Trees are associ- ated with improved visual quality of roadsides (Wolf 2003) and positive judgments of community character (Wolf 2006). Drivers encountering natural roadside views display reduced physiological stress response compared with those viewing built settings (Parsons et al. 1998). Reports of speed reduc- tions or traffic calming are of great interest and have some empiric support (Godley et al. 1999; Rosenblatt Naderi et al. 2006). A study in Toronto, Canada, found that street land- scape improvements reduced accidents by 5% to 20% (gen- erating significant public costs savings) and boosted pedes- trian use of urban arterials (Rosenblatt Naderi 2003). Context Sensitive Solutions An emerging design policy can provide opportunities to bet- ter integrate trees in roadsides. In recent years, citizens and communities have voiced concerns that narrowly defined transportation design generates external decisions that have high local impact (Passonneau 1996). Local protests and legal ©2006 International Society of Arboriculture challenges have caused costly project delays. In response, national and state transportation agencies have begun to implement practices of flexible highway design and Context Sensitive Solutions (CSS) (also referred to as Context Sen- sitive Design [CSD]) in an effort to balance issues of concrete and community. Two guides supplement the Green Book: Flexibility in Highway Design (FHWA 1997) and A Guide for Achieving Flexibility in Highway Design (AASHTO 2004b). They pro- vide ideas, options, and examples of ways to design more environmentally friendly highways without compromising safety and mobility. The guides stress the importance of early public participation, identifying community interests, and creative thinking to achieve community friendly highway de- sign (Moler 2002). Urban applications of CSS are becoming more common (AASHTO 2006). METHODS The benefits of urban trees are offset by concerns about traf- fic safety in national roadside policy, and the “clear zone” concept has strongly influenced design guidelines and stan- dards at the municipal level. Roadside trees are fixed objects, contributing to the U.S. annual crash rate, but accident sta- tistics are reported as if the transportation landscape is ho- mogenous. Additional research is needed to better understand the particular incidence and conditions of tree-related acci- dents on urban streets so that design standards better reflect actual conditions. An exploratory analysis was conducted to serve as a start- ing point for new approaches and to discern future research needs. Using archival transportation collision data, a progres- sion of statistical analyses was carried out to evaluate two research questions: • What are the patterns of association involving trees and roadside crash outcomes? • Do such patterns differ between urban and rural areas? Year 2002 data from the General Estimates System (GES) database of the National Automotive Sampling System were used for this study. The data are collected by the U.S. Na- tional Center for Statistics and Analysis, a division of the National Highway Traffic Safety Administration (NHTSA), to identify traffic safety problems and conduct analysis of traffic-related programs. A subset of the 91 GES variables was used for analysis. Selection was based on which factors prior studies had found to be associated with roadside crashes as well as original hypotheses on such relationships. Selected variables included driver gender and age, alcohol consumption, posted speed, restraint use, and road characteristics such as curve geometry and number of travel lanes. Some variables needed for analysis were not present in the data set in a useful form. When possible, these were con-
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