By Kathleen L. Wolf, University of Washington
kwolf @u.washington.edu

Picture full, leafy branches arching out over streets and sidewalks. Trees lend their beauty to streetscapes, and we now know that they also generate extensive environmental, social and economic benefits. Meanwhile, transportation officials plan and design roads to achieve high levels of safety and traffic capacity at lowest cost. Intangible values of the roadside, such as community character and environmental systems, are sometimes overlooked, including the urban forest. Our recent research has analyzed national transportation safety data in order to bridge concrete and community. Better science can help us place trees in city streets more safely.

Crash and Injury Risk
A starting point is to review the full scope of traffic safety. Americans made 229 billion household vehicle trips in 2001. There were 6.3 million accidents in the U.S. in 2002 (the focus year for our study), more than 43,000 people died, and 13,000 were killed in single-vehicle crashes. What is the overall crash risk to a driver? If translated to multi-year trends, the average driver has a crash about once per decade, usually causing minor property damage. The corresponding rate for fatal crashes is about one per 4,000 years.

Fatality and injury outcomes are of interest. Most crashes (61%) result in no injury. Of those having injurious outcomes, 14% resulted in possible injury, 12% resulted in a non-incapacitating injury, 12% resulted in an incapacitating injury, and approximately 1% resulted in fatality.

Now consider tree crashes. The transportation industry regards a tree as a fixed object in the roadside (as are poles, signs and roadside barriers). Collisions with trees account for nearly 25 percent of all fixed-object accidents each year in the U.S., resulting in deaths of approximately 3,000 people, and making up about 48 percent of fixed-object fatalities.

The national crash data set lists 36 accident types. The four most common of these overall are car vs. car collisions (78.6%), rollovers (4%), collisions with poles or signs (2.1%) and collisions with trees (1.9%). While collisions with trees happen at the lowest frequency of the four, injury rates are higher. Sixty one percent of collisions with trees resulted in definite injury, while in 29% vehicle occupants were unharmed.

Urban vs. Rural
Transportation officials rarely distinguish between urban and rural settings when reporting tree accident data. Location appears to make a difference. The crash effects of nearby trees along high-speed, rural roadways are indisputable. County and township roads which generally have restrictive geometric designs and narrow clear zones account for a large percentage of the annual tree-related fatal crashes, followed by state and U.S. highways having curved alignments.

Our study found that more accidents occurred in rural areas (63%) than urban areas (37%), with implications for auto occupants. Accidents in rural areas are more injurious relative to accidents in urban areas, and all injury outcomes are more frequent in rural areas. Of all accidents in rural areas, 6.1% are collisions with fixed objects, compared to 3.8% of urban accidents. High-speed driving generally leads to higher injury accidents with roadside objects. The average speed of all accidents was 34 mph, while the average speed for tree collisions was 48 mph, perhaps due to the higher rural incidence of crashes.

Driver Behavior
Ultimately the source of the crash problem is the fact that drivers leave the road. Road conditions contribute. One study reported that the most common environment for an accident is a winding and rural road, with the vehicle leaving the road on the outside of curves. Many crashes occur on weekends and during late evening hours, and often involve excessive speeds.

General traits of drivers involved in off-the-road accidents are under 35 years of age, male, and under the influence. In nations with high auto use male traffic fatalities outnumber female fatalities by about a factor of two.

Behavior choices play a major role. Driver error is a factor that contributes to more than 95 percent of traffic accidents. Personal choices about travel speed, use of intoxicants, and not using seat belts may influence first, the vehicle leaving the road, and second, the outcome of any crash that may occur. Drunk driving accounts for as much as half of all traffic fatalities. Speed-related fatalities accounted for about 30 percent of all traffic fatalities in the past ten years. Belt use reduces a driver's risk of death in a crash by 42 percent.

Research
Transportation engineering guidelines are developed at the federal level and are often adopted (with some modification) by state and local agencies. Federal funds for local transportation projects may obligate communities to comply with national standards. Many guidelines for trees in urban roadsides have been extrapolated from rural accident data. Additional research is needed to better understand:

• conditions and factors of urban roadside accidents, such as roadside context and driver behavior
• benefits of trees in urban transportation, such as traffic calming and pedestrian safety
• development and testing of roadside tree guidelines, such as barrier technologies and urban clear zones

Context Sensitive Solutions is a transportation design process (being implemented at federal and state levels) that invites local input, and is an effort to better integrate community values with transportation needs. Science based solutions are the starting point for integrating trees into safe streetscapes.

Learn more about this research and related publications at:
http://www.cfr.washington.edu/research.envmind/transportation.html

Acknowledgement: Research support provided by USDA Forest Service, on recommendation of the National Urban and Community Forestry Advisory Council.