INC NEWS - Trees make safe streets

[pat carstensen]

http://www.newurbannews.com/

Research: trees make streets safer, not deadlier	 Courtesy of Eric Dumbaugh
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. The JAPA articles by 
Dumbaugh and Gattis can be found at: 
www.planning.org/japa/pdf/JAPADumbaugh05.pdf.

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.
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This article is available in the September 2006 issue of New Urban News, 
along with images and many more articles not available online. Subscribe or 
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