Critical Width

Here are two shots of Chandler Blvd in Phoenix westbound, the first one is approaching 25th Street, and the second is at the intersection with 24th Street, going westbound.

Lanes that are wider than minimal width for a motor vehicle, yet not wide enough for a bicycle and motor vehicle to share side-by-side have come to be known as “critical width”, a.k.a. ambiguous width. This creates confusion for both cyclists and motorists, leading to unsafe passing.

The section of road between the Ray/Chandler split and 24th street is wider — as seen in the first photo. The section at the intersection is typical of the entire section beginning just east of 24th street and continuing out to Desert Foothills Parkway. (west of DFP is a real bike lane).

The section from 24th Street out to DFP is dominated by an oversized, landscaped median. By oversized, I mean that there is more than enough extra width to have made side-by-side shareable outside lanes, or real bike lanes — within the existing right-of-way. Instead, the city chose to install this odd configuration with “critical width”. (it is so large, it is easy to see it from the aerial/sat picture, my estimate is it’s at least 30 feet wide! But no room to have a shareable lane?)

Here are my measurements, where there are lines involved, I measured to the center of the line. Both sections have about a 16″ gutter pan that is not included in the following figures…

(left picture, above)  lane: 125" shoulder: 44"
(right picture, above) lane: 121" shoulder: 30"

If the edge stripe were not there, the usable space in the wider section would be 14’1″ — which would nominally be wide enough to share side-by-side (motor vehicle – bicycle) per AASHTO “green book”.

The narrower section, at 151″ (under 13′) usable, is not suitable for any general side-by-side sharing.

The changes in dimension are rather gradual. Which leads to yet more problems for cyclists who wish to share in the wider portion, and take the lane as it narrows.

It is also a signed Bicycle Route (see figure 2 for another shot of this section). The signage seems to be yet another source of confusion for ill-informed motorists.

And this is not an isolated design, the city inexplicably uses similar cross sections on other arterials, e.g. Ray Road.

Good graphics of how 14′ usable space can be shared by a typical cyclist and a typical, narrow, motor vehicle from cyclist and trafic engineer Richard Moeur. Also see similar diagram on this page at humantransport.org show typical widths for passing with typical vehicles, e.g. a “normal” SUV is about 7 feet wide including mirrors.

A possibly better solution would be to remove the stripe, since it doesn’t belond there anyway, and adding Sharrows (since somebody at the city obviously feels we need some sort of bike facility — these are already signed “Bike Routes”). Unfortunately, besides being “experimental”, according to guidance sharrows are not to be used on roads with posted limits above 35mph — so they are virtually prohibited in car-crazed suburban Phoenix. Well, except perhaps on roads that don’t need them, witness the many 35mph oversized laned collectors with bicycle lanes.