Archive for November 12th, 2008

Are Narrower Lanes More Dangerous?

An interesting article in the St. Louis Post-Dispatch notes that crashes have increased on I-44 after they were narrowed to 11 feet wide, from 12 feet. They were narrowed to add a fifth lane and increase capacity in the wake of the temporary closure of another road, Highway 40. This is of interest because a key congestion-fighting measure that has emerged for an era of scarce highway funds is increasing the efficiency of existing infrastructure (e.g., by carving out new lanes).

The article notes: “The increase in overall collisions is typical whenever lanes become narrower, according to the Federal Highway Administration. The skinnier the lane, the less room there is for error. It’s why the federal government recommends that lanes on interstates be 12 feet wide.”

But there are some caveats to consider, within the same article:

1.) “Volumes on I-44 have gone up 10 percent to 30 percent during rush hour since the Highway 40 closures, according to the Transportation Department.” If the crash figure is 27 percent higher than the comparison period, it’s hard to know what percentage of crash increase has to do with narrower lanes, and which has to do with increased volume (or some other factor).

2.) Fatal crashes have actually gone down. Does that make the entire facility more or less safe?

3.) The speed limits have been reduced, but driver compliance is an issue. “You look at them and think, ‘OK, there goes another idiot,'” Stremlau said. “It’s that way on any highway. It’s just exacerbated by the narrower lanes.” Safety depends on context, and the real issue seems to be driver speed rather than lane width.

4.) The types of crashes mentioned, multi-car rear-end collisions, have little to do, theoretically, with lane width. “The bulk of them,” a police officer said, “are caused by tailgating.” Tailgating is a speed issue, not an issue of proximity to a neighboring vehicle. The police even came up with a novel suggestion: “Despite more traffic, police say the additional lane can make the interstate feel less congested, thus encouraging drivers to floor it.”

5.) There may be less room for error, but that may make drivers pay more attention, thus canceling out the increased risk.

Ezra Hauer has researched this issue rather extensively, and reports on the complexity of the issue here. Note that some crash types have been found to increase over 11 feet.

Of one study, he writes: “They also note that “narrowing of lanes to 11 feet (or occasionally 10.5 feet) while maintaining shoulders did not change accident rates.” Based on the review of several projects in California the authors note that: “. . . higher accident rates had not materialized several years after lanes were narrowed and left shoulders were removed . . .”

Posted on Wednesday, November 12th, 2008 at 6:02 pm by: Tom Vanderbilt
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November 2008

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