Arboriculture & Urban Forestry 32(4): July 2006 175 Crashes and Safety In absolute terms, trees do pose a risk to drivers, yet the overall incidence of tree-related crashes and injury are rarely communicated within the broader context of U.S. driving behavior. What are the general traffic safety patterns? Accidents totaled 6,316,000 in the United States in 2002; more than 43,000 people died, and 13,000 were killed in single-vehicle crashes (NHTSA-FARS 2003; NHTSA 2004). If translated to multiyear trends (Evans 2002), the average driver has a crash about once per decade, usually causing minor property damage. The corresponding rate for fatal crashes is approximately one per 4,000 years. Factors determining traffic safety can be classified broadly into two groups: those related to driver behavior and those related to engineering, whether of roads or motor vehicles. Roadway and vehicle engineering have generated many ef- fective countermeasures such as vehicle body design and roadside barrier design. Behavioral factors that determine an individual’s risk in traffic are (1) an individual’s behavior and (2) the behavior of other road users. Personal choices about travel speed, use of intoxicants, and not using seatbelts have great influence over first, the vehicle leaving the road, and second, the outcome of any crash that may occur in the roadside. Drunk driving is a major safety problem, accounting for as much as half of all traffic fatalities (Evans 2002). The National Highway Traffic Safety Administration (2004) provides additional details. Speed-related fatalities (meaning travel speeds that exceed posted speed limits) ac- counted for approximately 30% of all traffic fatalities each year for the past 10 years. Seatbelt use reduces a driver’s risk of death in a crash by 42%. Although drivers routinely violate safe driving laws (such as speed limits and drunk driving), changes in driver behavior brought about by legislation have led to large reductions in casualties. Trees and Safety To summarize the findings of this study, tree collisions num- bered approximately 1.9% of all traffic accidents in 2002. Forty-six percent of these were severely injurious or fatal. Of 229 billion household vehicle trips taken in the United States in 2001 (as estimated from U.S. Census and FHWA data), approximately 141,000 included crashes with trees. Some- what more than one third of such accidents occur in urban areas. Trees, guardrails, and utility poles are all fixed objects associated with a high incidence of injury, whereas vehicle rollovers also pose high injury risk. Among the three object types, trees have the highest severity index for collisions both with and without airbags (Council and Stewart 1996). Ve- hicle impacts with trees are concentrated, which may highly deform vehicles and lead to higher incidence of injury. Injury severity has been consistent across studies. A Con- necticut study (Zuckier et al. 1999) found that trees are one of the leading causes of death by people striking fixed objects. More crashes occurred with guardrails, curbing, and utility poles (compared separately), but injury rates are higher in crashes with trees. Urban versus Rural Conditions Few studies have distinguished urban from rural conditions when assessing tree crash rates and outcomes. Patterns of statistical association in this study lead to these general con- clusions: (1) roadside crashes are more frequent in rural areas than in urban areas, (2) collisions with fixed objects are more frequent in rural areas, and (3) crashes occurring in rural areas are generally more harmful than those in urban areas. The few sources of urban/rural breakout data are generally consistent. In 1997, the U.S. fatal accident rate was less for urban areas (0.05 accidents per million vehicle miles [mvm]) than rural areas (0.07/mvm). Seventy-seven percent of fatal incidents involving trees occurred on rural roads in 1999 (Neuman et al. 2003). Studies in Michigan (Zeigler 1986) and Alabama (Turner 1990) found that urban tree crashes were lower injury risk situations than rural accidents. The NHTSA (2004) reported that the majority of traffic crashes (67%) occur in rural areas, and the distribution for tree crashes is nearly the same. When compared with the distribution of U.S. annual household vehicle travel miles, an inverse relationship is observed (Figure 4). Urban travel miles were approximately 62% of the total in 2002, or 1.6 trillion miles; rural vehicle travel miles totaled 975 billion or 38% (NHTSA 2004). More information is needed about urban situations, be- cause both driving distances and conditions differ in cities as compared with rural roads. As an example, the NCHRP (2002) comprehensive guide to Context Sensitive Solutions Figure 4. Urban/rural distributions of U.S. travel and acci- dents (%). ©2006 International Society of Arboriculture
July 2006
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