Arboriculture & Urban Forestry 32(4): July 2006 173 structed (using collapsed categories or creating indexes) and included an urban/rural breakout, accident types, and injury severity. The data set contained no explicit measurement to distinguish urban from rural settings. A dummy variable was constructed using five indicators, including population of ac- cident area (>50,000 for urban), number of travel lanes (four or fewer), and posted speeds (less than 72 kph [45 mph]). An accident category variable was collapsed into three dummy variables: collision with a nonfixed object (primarily other vehicles), collision with a fixed object (including trees), and noncollision accident (such as coming to a stop in a ditch). ANALYSIS AND RESULTS Analysis started with a reconnaissance of the entire 2002 data set and then descriptive evaluations of selected variables to better understand their scope. Subsequent analyses involved greater complexity and predictive capacity and revealed cer- tain limitations of the data set with respect to the research questions. Three sets of findings are presented: descriptive, comparative, and predictive. Descriptive Analyses The GES data set defines three general accident categories. These include collisions with nonfixed objects (85.2% of all accidents), noncollision accidents (4.7%), and collisions with fixed objects, including trees (10.1%). Considering only ac- cidents involving collisions with fixed objects, the top two objects struck are poles and signs (21%) and trees (19%) followed by guardrails (11%), ditches (11%), and traffic bar- riers (10%). The GES data set enumerates a total of 36 accident types. The four most common of these overall are car versus car collisions (78.6%), rollovers (4%), collisions with poles or signs (2.1%), and collisions with trees (1.9%). Fatality and injury (F&I) outcomes are of great interest. For all crashes, the majority (61%) resulted in no injury. Of those having F&I outcomes, 14% resulted in possible injury, 12% resulted in a nonincapacitating injury, 12% resulted in an incapacitating injury, and approximately 1% resulted in fatality. Crash locations are a key issue of this project’s inquiry. Many more accidents occurred in rural areas (63%) than ur- ban areas (37%). It is important to note that the GES data contain no detailed data regarding vegetation collisions such as trunk size, dis- tance from the road edge, or vegetation densities, making detailed characterization of accident conditions impossible. Comparative Analyses Comparative analysis examined whether a difference exists between two groups across some measure. Tests were struc- tured for two-tailed tests using 2 analysis or independent samples t-tests. Based on the research questions, the rate and outcomes of automobile collisions with trees were examined. One notable difference between tree collisions and all crashes is that of speed. The average speed at which all accidents occurred was 55 kph (34 mph). The average speed at which drivers struck trees was 77 kph (48 mph), a statistically significant differ- ence (t 23.94, P < 0.01) that perhaps reflects the higher rural incidence of crashes. The proportion of tree collisions by urban and rural areas was nearly the same as the rate for all accidents. Thirty-nine percent of tree collisions occurred in urban areas, whereas 61% occurred in rural areas. Most tree crashes occurred on undivided roadways (48.8%), most commonly having two lanes (40.3%), where the average posted speed limit was 84 kph (52 mph). Road traits are consistent with the finding that there is a higher probability of crashes with trees in rural areas. Tests of injury severity were also done. Certain crash cat- egories result in more serious injuries than others (2 7,384, P < 0.01). Noncollision accidents (such as rollovers or driving into a ditch) are the most injurious followed closely by collisions with fixed objects. Collisions with nonfixed objects (i.e., vehicle-to-vehicle impacts) are by far the most common accidents, but are also the least injurious. Collisions with fixed objects such as trees are often harmful because impact forces are greater when an object is stationary. When examining injury severity in more detail, it was found that car versus car was both the most common and least injurious crash situation (Figure 1). Over 63% of all accidents of this type result in no injury, whereas 11% result in serious injury or fatality. By contrast, rollovers are less frequent but result in F&I at a much higher rate. In terms of the two fixed-object crash types, striking a pole or post is generally less injurious than striking a tree. Although collisions with trees happen at the lowest fre- quency of these four major accident types, injury rates are Figure 1. Frequencies of injury severity by accident type (%). ©2006 International Society of Arboriculture
July 2006
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
