PPW Plaintiffs Cherrypicked Data to Attack DOT’s Bike Lane Evaluation

Opponents of the Prospect Park West bike lane got some page views in the Park Slope Patch yesterday, repeating arguments from their lawsuit against the city [PDF] (arguments that have also been published, basically unaltered and unanalyzed, in the Daily News and the Post). Their core claim is that DOT should not have used three-year averages to assess the rates of crashes and injuries before the bike lane was installed, because there was a small uptick in crashes and injuries if you compare the second half of 2009, before the bike lane was installed, to the second half of 2010.

If you’ve ever taken a course in statistics, though, you can tell that the plaintiffs are actually guilty of what they accuse the DOT of doing. The plaintiffs are cherrypicking crash and injury numbers to make it look like DOT selectively used data.

“It is the opponents’ lawyers who are grasping at aberrations and doing the very thing they accuse the DOT of — selectively picking data to stack the deck in their favor.”

Compared to the three-year average of crash and injury data used by DOT to assess safety before the project was installed, the opponents are using a smaller and less reliable data set. DOT was following established practice in using multi-year averages to assess injury and crash rates before the redesign — see this federal compilation of traffic-calming studies [PDF] and this NYCDOT project assessment [PDF] for other examples.

The plaintiffs’ year-over-year comparison does not meet established standards of safety evaluation in the transportation engineering profession. The data is too scant: We’re talking about four injuries in the 2009 “before” period, compared to five injuries in the 2010 “after” period (four of which were sustained in the same crash, and none of which involved pedestrians).

When you look at a more robust “before” data set — the three-year average used by DOT — the rate of injuries and crashes on PPW has declined since the redesign. Ideally, experts say, we could compare crashes and injuries from several years before the redesign was implemented to the same time frame after the redesign. But waiting several years to make the Prospect Park West project permanent would be negligent, knowing that the redesign has substantially reduced speeding, and that lower vehicle speeds are proven to improve safety. Project opponents conveniently omit the reduction in speeding on PPW when making their attacks.

Another rhetorical tactic favored by bike lane opponents is to call for “objective analysis.” So don’t take my word for it. Here’s what transportation planners who had nothing to do with this project had to say about measuring the safety impacts of street redesigns.

Dan Burden is the founder of Walkable Communities, Inc. and served 16 years as bicycle and pedestrian coordinator for the Florida Department of Transportation. He told Streetsblog:

A normal way to compare rates is to use the most recent data going back three years, or five, if you have that data, then averaging. Considering that the number of bicyclists went up post-construction, then it is important to consider the new count versus the old count when comparing crash data. I think [the crash] numbers are only one small part of the total gain… more people are bicycling, fewer people are speeding.

Gary Toth directs transportation initiatives at Project for Public Spaces. He is also a 34-year veteran of the New Jersey Department of Transportation. His take:

When DOTs analyze crash data, they avoid picking one year because there are always peaks and spikes caused by year-to-year happenstances. For instance, it could have been bad luck that a couple extra crashes occurred in any one year. So, it would be logical for NYCDOT to look beyond one year.

Almost no agency would try to reach conclusions based on one year’s worth of data — it is the opponents’ lawyers who are grasping at aberrations and doing the very thing they accuse the DOT of — selectively picking data to stack the deck in their favor.

Rachel Weinberger, a civil engineer and professor of transportation planning at the University of Pennsylvania, said:

Three years is the minimum you want, because any one year could be anomalous.

[A six-month study period] is insufficient to draw conclusions about crashes and injuries, but you have to at some point make a decision, and the positive outcome on speeding is tremendous and unequivocal. Why? Because the DOT has the opportunity to observe vehicle speeds every minute. Whereas if you’re counting crashes, because they’re such rare events, you need more data to make clear statements.

An experienced transportation professional, who specializes in bike and pedestrian projects and asked to remain anonymous because his firm is under contract with the city, said one year-over-year comparison is “absolutely not” the right way to measure safety effects. He added:

There’s only six months of “after” data, so it’s premature or speculative to reach conclusions [on crashes and injuries]…

But you can’t wait for statisticians. Loads and loads of research shows speed reduction is good. If speeds are reduced, injuries will come down.

The plaintiffs, he said, are like a patient who receives a ten-day prescription from a doctor to cure an illness, then sues the doctor after two days because not all the symptoms have disappeared.

Noah Kazis contributed to this post.

  • J

    Keep it coming, Ben. This pretty much destroys any data-based argument they ever had. I do agree with the writers that you can’t really assess safety (in terms of crashes and injuries) after only 6 months. However, you can assess perceptions of safety, and if people honestly felt that the design was unsafe, there’d be a lot more than 11 people that signed up to speak at the recent CB meeting.

  • Nice work, Ben.

    Meanwhile, this afternoon on the “controversial” Prospect Park West bike path…

    I decided around 2:15 to go out and get some lunch, and figured since it was such a nice day that I’d take PPW over to Flatbush. I decided to ride south first to the end and double back, since it was 60 degrees+ of sunshine. In my 10 minutes or so on the path, I encountered about 25 cyclists (quick “standard multiplier effect” application projects to well over 1,000 for the day), a mix of commuters, errand-runners, delivery people and one guy seemingly coming from a tennis match over at the Parade Grounds. There were a fair number of folks taking in the sun on a park bench or strolling, free of bike traffic, on the park sidewalk. There was a decent volume of traffic, which flowed steadily and smoothly, undeterred by a DOT crew filling potholes, with nary a blown horn. One fellow on a bike politely ran his bell when a young woman crossed the path against the light, and a couple people nodded appreciatively when I yielded and allowed them to cross. Don’t know who had the light, but it didn’t matter. I encountered a couple of runners and an older woman in a wheel chair being pushed down the path.

    It was such a nice scene that I forgot for a few moments that there was a lawsuit hanging over all of it.

    At Grand Army Plaza, I decided to follow the rules and bike counter-clockwise around the Plaza to get to northbound Flatbush. Waiting for lights, it took about three minutes and fifteen seconds to make the circuit. I encountered a very Beautiful Godzilla and another fellow salmoning on Plaza Street East, which I didn’t begrudge them since it was taking me about four times as long to go around to Flatbush as it would have if the planned two-way cycle track around Plaza Street had been implemented already.

    I won’t be surprised if cyclist numbers double on PPW from last summer’s huge jump. If I didn’t know better, I’d hardly have thought it was “controversial” at all.

  • car free nation

    I took the PPW bike lane this morning on the way to work at around 9 am. It was fairly empty of both cyclists and pedestrians, but there was a mom and her 7 year old on a scooter. It’s such a peaceful way to get to work.

    I didn’t see any pedestrians near the path today, but I often see dog walkers crossing the street. I give them the benefit of the doubt, and stop well before I would really need to, and I am often thanked. It helps, I believe, that I’m on a tandem, and often have a 5 year old on the back.

    As usual, there was very little car traffic, although I did see the yellow buses backed up at Poly Prep (I believe that’s the school the NIMBYs referred to) It wouldn’t be a problem if the parents didn’t drive their SUVs to drop their kids off in the no standing zone. I can’t see how the bike lane contributes to that problem.

    I agree with Eric. If I didn’t know there was a controversy, it would seem like it was a beautiful, permanent addition to the park.

  • That’s great news, Eric. Count yourself lucky to live in a neighborhood with such strong support of biking initiatives. On my little local ride this morning to the newsstand, bike store, bagel shop, park bench, library, and bakery I encountered zero other indolent bikers and at least two cars going through crosswalks in reverse.

  • “I decided to ride south first to the end and double back…”

    Eric! Don’t you understand that NBBL will now claim that radical pro-EBL lobbyists are trying to run up bike counts by making multiple passes on the lane?

  • Good point, Doug. And the cyclists I encountered were not all clustered around the 14 PPW spycam, so many of them actually didn’t count, I guess.

  • Joe R.

    If it wasn’t so far from me I’d ride down there just to be “counted”. In fact, I’d make loops of the park for an hour or two to get counted multiple times. 🙂

  • Oh, I forgot to mention that after I got home, and sat on my stoop to eat my lunch in the sunshine, it took all of about two minutes to be affronted by some guy screaming four-letter words at the driver ahead of him, who apparently was being too cautious in passing a double-parked dump truck. Downright bucolic.

  • The other day a colleague mentioned to me the same thing, that he was asked to provide expert witness for the plaintiff on this case but then they didn’t want him after he told them he wouldn’t filter through the data like they wanted him to.

  • BicyclesOnly

    Jim,

    You didn’t swear your prospective experts to confidentiality before interviewing them? You’re slippin’!

    Must be tough finding an expert with serious alternative mode credentials for this project, huh? Not much to offer experts by way of soft quid pro quos to induce them to work “pro bono.” Not to mention the fact that an expert would have to play charlatan to endorse NBBL’s analysis. I guess that’s partly what the NBBL legal defense fund is for…

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