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Archive for December 23rd, 2010

On Bike Lanes, Road Widths, and Traffic Safety

There was an assertion made in one of the letters (signed by Louise Hainline, Norman Steisel, and Iris Weinshall in response to a recent New York Times editorial on cycling that caught my eye:

When new bike lanes force the same volume of cars and trucks into fewer and narrower traffic lanes, the potential for accidents between cars, trucks and pedestrians goes up rather than down. At Prospect Park West in Brooklyn, for instance, where a two-way bike lane was put in last summer, our eyewitness reports show collisions of one sort or another to be on pace to be triple the former annual rates.

The first point is that while the PPW conversion did take away one travel lane, the width of the existing lanes was not altered. So there may be fewer lanes, but they are not, as the letter argues, “narrower.” It may be that entire street feels narrower, which, as an emerging school of what I’ll call ‘behavioral traffic calming’ argues, is actually a good thing. Drivers, as I’ve quoted Ezra Hauer as saying, “adapt to the road they see.” They either do not see traffic signs or fail to read their meaning correctly. If they see a wide open, long boulevard, they will drive like this.

Even if the lanes were narrowed, as John LaPlante recently argued in the journal of the Institute for Transportation Engineers, “there is no significant crash difference between 10-, 11-, and 12-foot lanes on urban arterials where the speed limit is 45 mph (or less).” (a finding, he notes, that was unfortunately left out of AASHTO’s recent Highway Safety Manual).

And there’s something deeply suspicious about that “eyewitness reports” note; were they actually out there, day after day, meticulously logging conflicts and crashes (tellingly, they make no note of severity)? And why, if everything was so great with the street before, were they even doing these “before” counts? As the case of roundabouts shows, what people perceive as individual danger often does not translate at all to an increase in overall risk; in fact, it’s quite the opposite.

But let’s take that notion — that fewer and narrower lanes lead to more crashes. This is a staple of traffic engineering, and in fact it does have some validity — when applied to highway environments (which PPW at times unintentionally resembles). Even here, though, the effects are often not ‘statistically significant’ and ‘more complex than expected.’

But in non-highway environments, there’s all kind of evidence that reducing the number of lanes (a.k.a. the ‘road diet’) can have positive safety benefits. As the Federal Highway Administration has noted:

Road­ diets­ can­ offer­ benefits ­to­ both ­drivers ­and­ pedestrians… road diets may reduce vehicles speeds and vehicle interactions, which could potentially reduce the number and severity of vehicle-to-vehicle crashes. Road diets can also help pedestrians by creating fewer lanes of traffic to cross and by reducing vehicle speeds. A 2001 study found a reduction in pedestrian crashing risk when crossing two-and three-lane roads compared to roads with four or more lanes.

But what if one of those lanes your crossing is a bike lane? Surely that must make things less safe, no? More interactions in less space. In a forthcoming paper to be published in the Journal of Environmental Practice Norman Garrick and Wesley Marshall examined 24 California cities (12 with relatively low traffic fatality rates, 12 with relatively high rates). They found that the cities that had a higher bicycle usage had a better safety rate, not just for cyclists but all road users. They write:

Our results consistently show that, in terms of street network design, high intersection density appears to be related to much lower crash severities. Our street design data also contains strong indications of these trends; for example, the high biking cities tend to have more bike lanes, fewer traffic lanes, and more on-street parking. At the same time, large numbers of bicycle users might also help shift the overall dynamics of the street environment – perhaps by lowering vehicle speeds but also by increasing driver awareness – toward a safer and more sustainable transportation system for all road users.

And as Eric Dumbaugh, of the University of Texas A&M, notes, “most recent research reports that wider lanes on urban streets have little or no safety benefit, at least to the extent that safety is measured in terms of empirical observations of crash incidence” (e.g., Potts, I.B., Harwood, D.F., & Richard, K.R. (2007). Relationship of Lane Width to Safety for Urban and Suburban Arterials. Transportation Research Board 2007 Annual Meeting; Milton, J., & Mannering, F. (1998). The relationship among highway geometries, traffic-related elements and motor-vehicle accident frequencies. Transportation 25, 395–413; and so on).

But the authors of this letter are not trafficking in empirical evidence, even as they allege that the NYC DOT’s data “more puzzlement than enlightenment.” It’s unfortunate that this letter is signed by a former DOT commissioner, and an academic — who should both know that it is evidence and analysis, not vague “eyewitness” reports and random testimony, upon which good science, planning, and safety interventions are made.

And as always, curious to hear of more work either supporting or countering what I’ve said here.

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Posted on Thursday, December 23rd, 2010 at 12:47 pm by: Tom Vanderbilt
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Traffic Tom Vanderbilt

How We Drive is the companion blog to Tom Vanderbilt’s New York Times bestselling book, Traffic: Why We Drive the Way We Do (and What It Says About Us), published by Alfred A. Knopf in the U.S. and Canada, Penguin in the U.K, and in languages other than English by a number of other fine publishers worldwide.

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