Charles St

Was this narrow space next to the curb considered a bike lane in this study?

“Evaluation of Wide Curb Lanes as Shared Lane Bicycle Facilities” is a 1985 report that resulted in AASHTO’s 1999 Guide for the Development of Bicycle Facilities stating in reference to the width of wide curb lanes (p. 17): “With these exceptions in mind, widths greater than 4.2 m (14 feet) that extend continuously along a stretch of roadway may encourage the undesirable operation of two motor vehicles in one lane, especially in urban areas, and therefore are not recommended.”

This alleged doubling up of motor vehicles in one lane is often cited in more recent bicycling research and in other design publications, so it deserves scrutiny.

Critique of: “Evaluation of Wide Curb Lanes as Shared Lane Bicycle Facilities” shows how this “research” is an early and seminal example of the junk science that pervades the bicycling literature.

3 Responses to “WCLs for Sharing”

  1. Bruce Epperson Says:

    Thanks for posting a copy of the report, which is hard to get. I had been seeking a copy to compare with the 1974 Jilla report and the 1975 Kroll and Ramey report and the 1977 Loop and Layton paper. The results in all three are about the same, that for level of service calculations on uninterrupted segments, a cyclist comprises a perturbation in a smoothly flowing traffic stream up to about the 17 or 18 foot width, and that a lane marking doesn’t make much of a difference.

    In a two-lane stream, the perturbation would be calculated the same as the impedance on a two-lane, two-way highway, using the prevailing flow, the speed of the obstruction, the oncoming gap, and the gap acceptance of the passing driver. For a multi-lane street the dominant factor would be the speed and density in the same-direction adjacent lane, the speed of the obstruction and the gap acceptance probability of the passing driver.

    The earlier studies seem to have been the only ones consulted prior to the development of the 1981 AASHTO Bicycle Guide, and the McHenry study added prior to the publication of the 1991 update of the Guide and the bicycles chapter of the 1994 Highway Capacity Manual. All seem to point in the same direction: that the AASHTO Guide and the HCM were developed with a strict focus on perturbation removal, i.e. on the maintenance of smooth channel flow on uninterrupted segments. How the Per Car Equivalents (PCE) were derived in the HCM from this body of work remains a mystery.

  2. Wayne Pein Says:

    I’m glad to have been given the paper and to have put to out with critique. This paper has done incalculable damage.

    • Bruce Epperson Says:

      Oh, I don’t know. I think you may be exaggerating its importance. After all, it says basically the same things as the earlier studies: the narrower the lane, the closer to right hand edge of the road the average cyclist rides, and the higher the probably for lane displacement by cars. That’s what the Highway Research Board wanted to know: how much of a flow obstruction does a bicycle present? At first, it was feared that even if you gave bicycles 16, 17, 18 feet the mere presence of the bicycle would cause the cars to slow and would send ripples back in the traffic stream on a C-D road. Nah. So then speed differential was dropped and everyone focused on displacement.

      It also agrees with the later studies, that bike lanes don’t make all that much difference, in that lane displacement occurs all the way up to the 16-17+ foot range. It was quite valuable in that it was the first study to take a crack at the differential in lane displacement on multi-lane streets between when there was and was not a car (actually a stream of cars) in the center, same-direction lane, even if it was a bit muddled in its execution.

      It was one of only about four or five studies that was actually considered in drafting the 1994 Highway Capacity Manual bike chapter (the others being the studies referenced – Jillan, Loop & Layton, Kroll’s ASCE paper, etc.). It is one of the best kept secrets that the 1981 and 1991 AASHTO Bike Guides – at least for the on-road facilities – were based on the HCM work, which, in turn, was based on smoothing arterial flow by keeping bikes from perturbing flow at mid-blocks and (especially) at intersections. That was always a bit of a stretch. Nobody was able to show that you prevented flow perturbation by bikes in intersections by increasing approach width. That methodology was abandoned in 2000 in favor of Botma’s mutual-interference model.

      If you still have the original in digital format, I would appreciate it if you could email it to me at I would like to make a paper copy. I keep an archive of the old, first generation research work. Some if it is so hard to get.

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