Sunday, March 13, 2011

Quantitative bike lane evaluation

As I've made clear, I am very much in favor of the Prospect Park West bike lane. In my view, the city has demonstrated that it has improved safety tremendously. The opponents do have a minor safety issue: the increased density of the cars on Prospect Park West means that people boarding, exiting or unloading cars on the avenue are more likely to be "buzzed" by cars. This may feel less safe, but it may be compensated for by the fact that the cars are going slower. In any case, the reduced speeding has made the avenue safer for pedestrians, cyclists and car occupants, so there is an overall increase in safety.

Unfortunately, there's a problem with the data demonstrating this, and the bike lane opponents have seized on it in their SLAPP lawsuit, which you can read in this PDF helpfully hosted by Streetsblog. The number of crashes resulting in injuries on the avenue have actually increased by twenty percent since the lane was installed, according to the Department of Transportation's data. Rather than acknowledging that, the DOT has preferred to compare last year's data with an average of the three previous years. The bike lane opponents argue that the decline in injuries was actually due to retiming of the traffic lights, and that the bike lane increased injuries.

The bike lane opponents are wrong, for three reasons. The first has to do with measurement. Some injuries are not reported to the police, and it's possible that there were more injuries in 2009, but they weren't all reported.

The second reason has to do with outside factors. The opponents do not mention that the twenty percent increase was from four injuries in a six-month period to five, a total increase of one injury. Things like injury rates always fluctuate over time due to any number of factors. That additional injury could have been due to the fact that a particularly reckless driver moved to Windsor Terrace that year, for example. It could be due to almost anything, which is why the ideal is to have a multi-year average before and after. It is even possible that the injury rates went down, but that last year there just happen to have been two or three more crashes resulting in injury.

There is also the effect of popularity to consider. When I lived in Park Slope, I avoided crossing Prospect Park West whenever I could. I spent somewhat less time in the park than I would otherwise have, and once I got in the park I stayed there until it was time to go home. It may very well be that more people are feeling safer crossing Prospect Park West, and are thus crossing it more. There is more safety per crossing, and more safety per person, but we aren't measuring those, we're only measuring overall safety per half year.

Now watch Councilmember Lander debate Jim Walden on New York 1. Lander is a charismatic young politician going up against a lawyer who looks like Agent Smith, so he's got an advantage already. But he squanders that advantage, because he will not acknowledge that injuries went up from 2009 to 2010. Walden picks up on that - and so does Errol Lewis, although he was pretty fair - and they both try to pin Lander down on it. Lander avoids the issue, and it makes him look like a shifty politician, hiding Things the DOT Doesn't Want You To Know.

There is no way to avoid that issue. The only thing to do is acknowledge it, respond to it and move on. Yes, unfortunately there was one more injury, and our sympathies are with the victim. It may be caused by the bike lane, but it's much more likely to be a reporting error, normal fluctuation, or increased use of the street by pedestrians. We won't know for sure until we've got a couple more years to measure. There's nothing to hide, we just don't have enough information at this time to say whether there's a real increase in danger due to the bike lane.

Sometimes the data says things you don't like. You have to own it. You can't look like you're fudging it or ignoring it. We need to maintain our integrity - and our appearance of integrity - in this fight.

1 comment:

Mike said...

One of the crashes in 2010 caused 4 injuries (all to car occupants). And just like that, the stats show an increase. Pure happenstance.