Arboriculture & Urban Forestry 32(4): July 2006 171 Geometric guidelines have focused on achieving the high- est levels of safety and capacity for a road at the lowest cost. Such goals have been accomplished by building wider lanes and shoulders along with straighter and flatter alignments. Engineering economics, a mainstay of engineering education and practice, focuses on solving problems based on benefit- to-cost ratio or lowest life cycle cost of solutions. Two issues emerge with regard to trees and the guidelines. First, transportation designers may fail to heed the flexibility implied and framed by the Green Book and implement rec- ommendations (and local derivations) as “standards.” Trans- portation officials are encouraged to mitigate the effects of environmental impacts using “thoughtful design processes” (AASHTO 2004a) as standards have been “less rigorously derived” for urban settings (AASHTO 2004b). Second, most geometric design criteria apply to high-speed and rural roads, so appropriateness of their use in urban areas is debated (McGinnis 2001). Engineers often take a conservative ap- proach to maximize safety and capacity (Otto 2000). Clear Zone Deterrence and mitigation are primary approaches to improv- ing roadside safety (Mak 1995). Deterrence emphasizes the importance of keeping cars on the roadway, whereas mitiga- tions reduce the severity of consequences when drivers leave the paved area. AASHTO’s approach to roadside safety has historically focused on mitigation, including removing, relo- cating, altering, and shielding hazards. The “clear zone” (also referred to as a “recovery zone”)is a primary crash mitigation approach. Clear zones are swaths of land of prescribed width adjacent to road edges that are clear of fixed objects that may damage a vehicle on impact, including trees and utility poles. Adequate clear zone enables an errant driver to safely return to the roadway or bring the vehicle to a safe, controlled stop. National clear zone policy, based on the concept of the “forgiving roadside,” emerged in a 1967 AASHTO report to address inconsistent practices across the states (Turner et al. 1989). The report standardized clear zone definitions and guidelines at the federal level, thereby providing a model for state and local agencies. The Green Book and the Roadside Design Guide (AASHTO 2002) are the key references for determining clear zone widths with primary applications being high-speed and rural roads. Engineering tables provide variable clear zone distances based on traffic volumes, speeds, and roadside ge- ometry, particularly shoulder slopes. Additional adjustment or correction factors are provided for particular road section types. Contextual elements such as pedestrian facilities or adjacent land uses are not included in calculations. A 9.0 m (29.7 ft) clear zone width is generally recom- mended for high-speed, high-volume roads with nearly level rights of way, whereas a minimum clear zone of 3.0m(9.9 ft) is recommended for low-speed roads. Clearance distances may be less if a fixed object is located behind a guardrail or other approved barrier. Also, the AASHTO transportation landscape guide (AASHTO 1991) lists conditions that can be “weighed to decide if a special exception is warranted,” in- cluding roads of historic or scenic significance, endangered species impact, adverse impacts of erosion or sedimentation, and significant negative changes in roadside character or aes- thetic values. Less distinct guidelines are provided for urban arterials, collectors, and streets, because the space available for clear zones is typically restricted, and travel speeds are more vari- able. For instance, a horizontal offset distance of 0.5 m (1.65 ft) beyond the face of the curb to the outside edge of a fixed object (such as the anticipated outside diameter of a mature tree trunk) is the minimum distance allowed for urban low- speed, local roads having a curbed edge. Urban arterials and collectors, usually of higher speeds, are recommended to have increased offset distances (AASHTO 1991). The gen- eral inclination is to favor wider clear zones. Tree Crashes Data on tree crashes is presented in a straightforward and consistent way in many transportation planning publications. Based on crash data analysis of the 1990s, single-vehicle collisions with trees account for nearly 25% of all fixed- object accidents each year in the United States, resulting in deaths of approximately 3,000 people and making up ap- proximately 48% of fixed-object fatalities (FHWA 1997; AASHTO 2002). Higher crash rates and fatalities are also associated with roadside utility poles and guardrails. The crash effects of nearby trees along high-speed, rural roadways are indisputable. County and township roads that generally have restrictive geometric designs and narrow, off- road recovery areas account for a large percentage of the annual tree-related fatal crashes, followed by state and U.S. numbered highways having curved alignments (AASHTO 2002). Existing trees often pose greater risk than trees that have been placed along new or reconstructed roads. Other than citing the need for large side clearance along high-speed roadways, most guidelines on geometric design are vague regarding design standards pertaining to trees (Sullivan and Jud 2001). An often cited rule of thumb, first posed in the 1960s for interstate highways (AASHTO 1961), is that a tree with trunk size of more than 100 mm (4 in) dbh is considered a fixed object and should be placed 6.0 m (19.8 ft) from the road edge. Later, vegetation management guide- lines placed such trees 9.0 m (29.7 ft) from the road edge (USDOT 1992). Alternative findings are rarely considered. For instance, Zeigler (1986) found that fatal accidents on rural roads were usually associated with larger trees, of 500 mm (20 in) or more in diameter, with nonfatal accidents associated with smaller trees. ©2006 International Society of Arboriculture
July 2006
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