street tree safety

I'm posting this as food for thought. I haven't gone through his article with a fine tooth comb, but I find his ideas interesting. If nothing else, it is good practice as individuals and as a profession to go back and evaluate our base assumptions once in a while. As my signature shows, even the venerable Mr. Eno made mistakes.

Safe Streets, Livable Streets

"While their model for rural areas performed as expected, with trees and other features being associated with a statistically significant increase in the number of roadside crashes, their model for urban areas produced radically different results. Not only were trees not associated with crash increases, but ... the presence of trees in urban areas was associated with a decrease in the probability that a run-off-roadway crash would occur."

Click to expand...

(Emphasis added)

I'm not sure the issue is settled yet (read the counterpoint after the references), but the article does have some interesting ideas. Most importantly, that we (the highway engineering community) can't use the same tools in urban areas that have worked well in rural areas.

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

Thank you for posting the "research" paper. I can't recall how many times I've seen statements in reports prepared by architectural firms that street trees make traffic slow down and are therefore a safety feature. The reports also usually recite from the "vehicle speed is evil" catechism.

The author appears to be determined to cast as much doubt as possible on existing safety studies to make his new paradigm appear reasonable. I think Gattis's discussion following the footnotes is a sufficient rebuttal.

I expect the we will be seeing reports recommending a "wholistic" design for the project.

All in all I think the article presents some interesting ideas and valid points. While the aurthor has an obvious agenda and appears to have cherry picked the data sets to make his point, that does not completely invalidate his points. I was unimpressed by Gattis's rebutal, he doesn't really say much.

As for the points in the original article, my own observations would agree that if you keep the drivers attention on driving, you will have fewer accidents. As for the corelation between street trees and other roadside developement keeping the drivers attention ... well it makes sense, but that does not make it a true statement. Additional study is needed to make that connection.

Personally, I believe that we design our streets for too high of a speed and that this higher design speed does make the area around the street less pedestrian friendly. Does this bother me on a 4 lane arterial, no; does it bother me on a 2 lane residential street, yes.

With respect to design speeds, I firmly agree with the auther that the design speed should be set based on the context of the street and then the design should be such that the speeds are limited by the design speed. The Green Book has a clause (sorry don't know where and I'm too lazy to go find it right now) saying that curves and such should be designed for 5 MPH over the design speed. All this does is encourage people to dive 5 MPH over the stated design speed since the road is really designed for the higher speed.

How about this: Vehicle speed is not evil. Inappropriate vehicle speed is evil. Fifty mph(80 km/h) on an urban street is as inappropriate as 20 mph(32 km/h) on an interstate.

I think we need a study on the order of Dr. C. Zegeer's study on marked vs. unmarked crosswalks, with matched paired comparisons, solid statistical methodology, etc. A forthcoming NHCRP report may provide some answers:
With current methodologies, we should expect to have conventional wisdom upset once in a while. After all, it wasn't that long ago when you'ld be ridiculed if you said having an explosive device in your steering wheel would make you safer. Now they surround you in many new cars.

A lot of things in traffic safety are counterintuitive. Installing a traffic signal often increases crashes. Roundabouts are safe. Lowering the speed limit has little effect on speed. Marked crosswalks do squat for safety on low volume roads, and are associated with increase risk to pedestrians on high volume roads.

As for me, I'll keep an open mind until I see something definitive.




"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust