by Tom Vanderbilt
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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). [Also note there is no difference in capacity for 10-12 lanes as well.]
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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).
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https://www.howwedrive.com/2010/12/23/on-bike-lanes-road-widths-and-traffic-safety/oldId.20110116070429798
Biking in Baltimore