Arboriculture & Urban Forestry 45(2): March 2019 Naderi (2003) found that inclusion of features such as trees and concrete planters along the roadside resulted in statistically significant reductions in the number of mid-block crashes along the sampled road- ways, with the number of crashes decreasing between 5% and 20%. Lee and Mannering (1999) also found that in urban areas, the presence of trees was associated with a decrease in the probability that a run-off-roadway crash would occur, and the opposite effect was found in rural areas. Park and Abdel-Aty (2015) found that safety measures such as wide shoulders and reduced speed limits had less effect on promoting safety as driveway density and pole density increased. It appears there is a body of research suggesting that a defined roadside boundary, as enhanced by roadside trees and other fixed objects, has a traffic-calming effect that enhances safety in some circumstances. The present study found that rural tree crashes were more frequent and more severe as compared to urban tree crashes, which may support the assertions of Dumbaugh (2005) and Harvey and Aultman-Hall (2015). The present study also found that tree crashes are most frequent at nighttime with lighting and least frequent at nighttime without lighting. Low visibility is similar to fixed objects in that they are both obvious hazards, which may induce drivers to reduce speed, thus lowering accident severity and frequency. Of course, the unique characteristics of the roadway and surrounding land use will impact driver perception of hazards. Ultimately, urban driving patterns differ from rural driving patterns, and this impacts both the frequency and severity of tree- and other run-off-road collisions. Holdridge et al. (2005) modeled injury severity in fixed object crashes and found that trees, utility poles, and the leading ends of guardrails and bridge rails increase the probability of fatal injuries in run-off-road crashes. Other variables that contributed to fixed-object crash severity include speed, intoxication, and falling asleep at the wheel/inattention. By contrast, the present study did not find a significant impact of intoxication on tree crash frequency. Implications for Planning In looking at all traffic accidents (i.e., not just single- vehicle), tree-related crashes accounted for 1.5% of all crash events recorded (n = 3,033,048) during the eight - year study period. While somewhat disproportionate given crash frequency, tree-related traffic crash fatal- ities accounted for just 3.5% of the total road fatalities recorded from 2006 to 2013 (FDOT Office of 71 Planning 2017). On average, 94 people died each year in tree-related car crashes. During the same time frame, there was an average of 15,464,241 licensed Florida drivers (FDOT Office of Planning 2017). Ignoring unlicensed or visiting motorists, this equates to an average annual risk of harm (based on fatalities) of 1:164,513 for tree-related, single-vehicle crashes. This calculated risk of harm assumes the driver is the only occupant. By comparison, the annual risk of harm for motor vehicle occupants in general in the United States is 1:9,008 (National Safety Council 2017). Interestingly, the annual risk of harm associ- ated with working in the finance and insurance indus- try in the United States is double (1:84,367) the risk of harm posed by Florida’s roadside trees (National Safety Council 2017). While risk assessment in the United States is largely qualitative (Smiley et al. 2011), arborists and urban foresters in the United Kingdom assess and manage tree risk by estimating risk of harm (Ellison 2005). Drawing on the Tolerability of Risk (ToR) framework (Health and Safety Executive 2001), the Quantitative Tree Risk Assessment method defines situations with an annual risk of harm 1:1,000,000 or less as being broadly acceptable (Ellison 2017). Situations, such as tree-related car crashes in Florida where the calcu- lated annual risk of harm falls between 1:10,000 and 1:1,000,000, are deemed tolerable to the public if the risk has been mitigated to be as low as reasonably possible (ALARP) given the costs and benefits of risk reductions efforts (Ellison 2017). Future research to quantify the costs of current roadside clear zones and relative changes in safety and management costs (and loss of tree benefits) for more or less aggressive man- agement scenarios could help determine if risks are currently ALARP. While potential risks such as second-hand smoke inhalation offer no benefit to those subjected to it, roadside trees differ in that they can do both harm and good. In fact, excessive tree removal has its risks. In a study on the effects of drastic urban tree removal following infestations of the highly destructive emer- ald ash borer, researchers found that areas that lost tree canopy over a 17-year period experienced an additional 6,113 deaths related to respiratory illness and an additional 15,080 deaths linked to cardiovascular- related deaths (Donovan et al. 2013). Even the act of removing trees itself increases the likelihood of death, as forestry is consistently ranked one of the most dan- gerous occupations (National Safety Council 2017). ©2019 International Society of Arboriculture
March 2019
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