Critical Width

Chandler Blvd, Phoenix. Westbound, approaching 24th StreetHere 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.

Chandler Blvd, Phoenix. Westbound, at 24th StreetThe 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 show typical widths for passing with typical vehicles, e.g. a “normal” SUV is about 7 feet wide including mirror (correction: WRONG — a, for example, Ford F-150 is just shy of 7′ with its mirrors FOLDED IN. With factory mirrors it is 97″ — over 8 feet wide!). Best graphics yet: Dan Guiterrez’s Facebook: “These slides show why edge riding in standard width lanes is hazardous, why lane control avoids these hazards, and how much width is needed to create bike lanes free of both types of hazards”.

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.

Dutch CROW and critical width

A British study referred to in a article said (3.65m is just a hair less than 12 feet):

A related issue the researchers identify is that standard UK traffic lanes are 3.65m wide, which puts them in what the Dutch CROW road design guidelines call the ‘critical’ category for combined bike and motor vehicle use.

Influence of road markings, lane widths and driver behaviour on proximity and speed of vehicles overtaking cyclists. Accident Analysis & Prevention (full text available here) Shackel, S. C. and Parkin, J. (2014)

The study finds it may be beneficial for bicyclist safety to not have centerline markings on certain roads.


ITE TCD Handbook

Another exposition  of the concept of critical width, and its attendant problems, can be found in the  ITE TCD Traffic Control Devices Handbook, 2nd Edition. Chapter 14 by Richard Moeur and John Ciarellia:

In locations where the curbside lane width is less than 11 ft. (3.3m), bicyclists and motor vehicle will usually occupy the same lane width, as the lane is perceived as being insufficiently wide for side-by-side passing. In locations where the lane width is between 11 and 14 ft. (3.3 and 4.3m), the perception of “shareable” width may be ambiguous, and bicyclists may ride to the extreme right of the travel lane to politely accommodate faster traffic.

This can create two operational and safety problems. First, the bicyclist may be riding in the gutter pan, next to a vertical curb, or near posts and other roadside objects, creating the risk of a fixed-object crash. Second, this can create the misleading impression the remaining unoccupied travel lane width is adequate for an overtaking maneuver, resulting in too-close passing inconsistent with good operating procedure or law, and may result in sideswipe-type crash if the motorist misjudges separation distance. In locations with 11 to 14 ft. (3.3 to 4.3m) lanes, it may be advisable, based on engineering judgement, to install additional devices, such as shared lane marking and BICYCLE S MAY USE FULL LANE (R4-11) signs, or if practicable, reallocate lane widths to reduce the ambiguity.

Note the curb lane width is “continuous usable width (not including gutter pan)”