Re: [CG] Another error in Teschke study
Tricia Kovacs
Thank you for pointing out this error in the re-analysis of the Wachtel & Lewiston study. I think you were referring to the re-analysis done by Lusk et al in the paper "Risk of injury for bicycling on cycle tracks versus in the street", Feb, 2011.
When I read the Lusk study, I didn't think it was right for the authors to just arbitrarily add 26% to the roadway crash numbers. The percentage 26% was the percentage of non-intersection crashes in the city of Palo Alto during the study period. I thought that the re-analysis should have used the actual crash data for the three roadways that were being studied (including intersection and non-intersection crashes). Thank you for pointing out the error in adding 26% which would include dooring crashes, which wouldn't have occurred on some portions of the roadways in the study.
Looking back at the W&L study, the types of non-intersection crashes in Palo Alto were listed in the report:
total bike/car crashes = 314
total non-intersection crashes = 81
hit from behind = 5
sideswipes = 8
other (bicyclist overtook parked car, motorist opened car door, motorist or bicyclist changed lanes improperly) = 68
So, we know that the number of non-intersections crashes that do not involved parked cars is somewhere between 4% (hit from behind + sideswipes) and 26%.
Maybe we could look at other studies to see what percentage of bike/car crashes are involved with parked cars, but then we would be doing the same thing that the Lusk study did, i.e. making up numbers.
One other point that is described in the W&L study is that their definition of intersection included "points where driveways, sidewalks or paths meet a roadway, or where sidewalks or paths meet a driveway".
The Wachtel & Lewiston study is very important to me because it is the study I always cite when I talk about the dangers of sidewalk or sidepath cycling. I feel that it is the best study of all the ones I've read about because it used actual cyclist counts and crash data, rather than some of the other studies which relied on surveys of cyclists.
Tricia Kovacs
P.S. I hope it's OK that I copied your email and my response to the bicycledriving forum.
On 3/20/2013 1:10 PM, John Forester wrote:
The Teschke paper on cycle tracks contains one more error, though not on
the direct line of the proof. Teschke et al criticize the Wachtel and
Lewiston study of car-bike collisions on the cycle tracks installed on
some Palo Alto streets. The criticism is based on the fact that W&L
counted only intersection collisions, producing the result that
cycle-track cycling had a 1.8 times higher rate than did roadway
cycling. Teschke et al claimed that this ratio should be corrected for
the non-intersection collisions that same-direction roadway cyclists
would have incurred. Teschke et al used Palo Alto's city-wide rate of
car-bike collisions to calculate this correction, assuming that
same-direction roadway cyclists would have incurred collisions that
cycle-track cyclists would not have. Teschke et al assumed that these
meant all others, including dooring collisions. However, it so happens
that none of the Palo Alto streets alongside which cycle tracks were
designated initially had had curb-side parking, so no dooring collisions
would have occurred. Also, none of the existing driveways was changed in
any way, so that it is most likely that the driveway collision rate
would be higher with the cycle track than without (two chances of
collision, rather than one).
In short, Teschke et al's criticism demonstrates again their ignorance
of the traffic circumstances.
--
John Forester, MS, PE
Bicycle Transportation Engineer
7585 Church St. Lemon Grove CA 91945-2306
619-644-5481 fore...@johnforester.com
www.johnforester.com
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