Research: trees make streets safer, not deadlier

By Robert Steuteville
Ithaca, NY (September 2006)- Proposals for planting rows of trees along the roads a traditional technique for shaping pleasing public spaces are often opposed by transportation engineers, who contend that a wide travel corridor, free of obstacles, is needed to protect the lives of errant motorists. Increasingly, however, the engineers beliefs about safety are being subjected to empirical study and are being found incorrect. Eric Dumbaugh, an assistant professor of transportation at Texas A&M, threw down the gauntlet with a long, carefully argued article, Safe Streets, Livable Streets, in the Summer 2005 issue of the Journal of the American Planning Association. A follow-up article by Dumbaugh, in the 2006 edition of Transportation Research Record, will present further evidence that safe urban roadsides are not what the traffic-engineering establishment thinks they are.


Though engineers generally assert that wide clear areas safeguard motorists who run off the roads, Dumbaugh looked at accident records and found that, on the contrary, wide-open corridors encourage motorists to speed, bringing on more crashes. By contrast, tree-lined roadways cause motorists to slow down and drive more carefully, Dumbaugh says. Dumbaugh examined crash statistics and found that tree-lined streets experience fewer accidents than do “forgiving roadsides”- those that have been kept free of large, inflexible objects. He points to a growing body of evidence suggesting that the inclusion of trees and other streetscape features in the roadside environment may actually reduce crashes and injuries on urban roadways.
Among the cases cited in his JAPA article are these:
* A study of five arterial roadways in downtown Toronto found that mid-block car crashes declined between 5 and 20 percent in areas where there were elements such as trees or concrete planters along the road.
* Urban village areas in New Hampshire containing on-street parking and pedestrian-friendly roadside treatments were two times less likely to experience a crash than the purportedly safer roadways preferred by most transportation engineers.
* A study of two-lane roadways found that although wide shoulders were associated with reductions in single-vehicle, fixed-object crashes, they were also associated with a statistically significant increase in total crashes. A rise in multiple-vehicle crashes offset the decline in fixed-object crashes.
* An examination of Colonial Drive (State Route 50), which connects the north end of downtown Orlando to the suburbs, found fewer serious mid-block crashes on the livable section than on a comparison conventional roadway. According to Dumbaugh, the conventional roadway also was associated with more injuries to pedestrians and bicyclists.
Drivers Adjust
In his explanation of why livable streets enhance safety, Dumbaugh says drivers are reading the potential hazards of the road environment and adjusting their behavior in response. Dan Burden, senior urban designer for Glatting Jackson and Walkable Communities Inc. in Orlando, notes that there is research showing that motorists need and benefit from tall vertical roadside features such as trees or buildings in order to properly gauge their speed.
What Dumbaugh advocates appears to be consistent with, though not as radical as, the work that traffic engineer Hans Monderman has been doing in small towns in Holland. Monderman has introduced trees, paving, stones, fountains, and other features, while eliminating conventional safety devices such as traffic lights, speed-limit signs, and pavement markings. Monderman discovered that, at least in small Dutch towns, drivers therefore slow down and become alert to clues about how to behave. JAPA accompanied Dumbaugh’s article with a counterpoint from J.L. Gattis of the University of Arkansas-Fayetteville, who argued that the studies cited are not conclusive. More context-sensitive research is needed, Gattis said.
Since then, Dumbaugh has written the forthcoming Transportation Research Record article, which reports on what Dumbaugh found when he examined safety on three routes- State Routes 15 and 44 in DeLand, Florida, and State Route 40 in Ocala, Florida- that have pedestrian-friendly designs along parts of their length and conventional designs along other sections. Dumbaugh discovered that the pedestrian-friendly segments experience 40 percent fewer crashes than comparison roadways.
Burden told New Urban News that many traffic engineers work out of a pseudo-science when it comes to trees and crash causation, and many others are not well tuned in to urban crash causation. Research like Dumbaugh’s may help overcome that failing. Burden has incorporated some of Dumbaugh’s findings into a new article, 22 Benefits of Urban Street Trees. Among the benefits Burden attributes to street trees are the abilities of tree canopies to reduce temperatures at pedestrian level, absorb some tailpipe exhaust, make drivers calmer, and extend the life of asphalt paving by 40 to 60 percent.
As a general principle, Burden urges that engineers, planners, architects, and landscape architects work closely with one another to come up with functional, safe, complete, and successful urban spaces. Meanwhile, he says, city councils and other community leaders need to exercise more control over important decisions about things like urban street trees instead of leaving such matters solely to transportation engineers.
Related Resources:
New Urban News
Safe Streets, Livable Streets
22 Benefits of Urban Street Trees