The dimension below all refer to urban streets with curbs and no parking… Most recommendations for Bike Lane dimensions refer to the AASHTO Guide for the Development of Bicycle Facilities , the general rule is
the minimum bike lane width is 5 ft (1.5 m), measured from the face of a curb or vertical surface to the center of the bike lane line
However, that changes when a gutter pan is involved, more width is required to meet their recommendations —
Along sections of roadway with curb and gutter, a usable width of 4 ft (1.2 m) measured from the longitudinal joint to the center of the bike lane line is recommended
The Institute for Transportation Engineers ITE TCD Traffic Control Devices Handbook, 2nd Edition. Chapter 14 by Richard Moeur and John Ciarellia has very similar guidance, referring to what the AASHTO guide calls “usable width” as “clear pavement width”:
Bike lanes should have a minimum of 4 ft. (1.2 m) of clear pavement width, excluding gutter pans, longitudinal joints, and other obstructions. A width of 5 ft. (1.5 m) or greater is desirable, especially in locations with curbs at the roadway edge. On roadways with higher motor vehicle speeds, a width of 6 ft. (1.8 m) or greater can make the bike lane more comfortable for use by less-confident riders.
So, usable width in a bike lane is exclusive of gutter pan width (as well as any other obstruction). The gutter pan width is excluded because the joint between the pan and roadway surfaces cannot be made perfectly flat, and furthermore over time they heave/deteriorate/move (see e.g. the condition of Warner Road east of Kyrene, finally repaired in 2016). Debris also naturally collects in the gutter, well it collects in the BL, too, the further from the most heavily traveled portion of the road the worse the problem is.
Four feet is a magic number because AASHTO refers to that as the “Minimum Operating” space of the standard adult bicyclist riding an upright bicycle, see their figure 3-1.
In the illustration above right, both scenes depict a nominal 4′ of usable space; in the top photo the space is next to a 17″ gutter pan (usual for metro phoenix area) and the bottom pic is next to a 24″ pan. So the total width is approximately 5.5′ (top) or 6′ (bottom) but both have only the nominal (the AASHTO guide calls it “recommended”) amount of usable width. [see here for some details about gutter pans]
Because the bike stencil in the photo above, and see MUTCD fig 9C-03 is 40″ wide, it’s pretty easy to eyeball a sub-standard width bike lane if you know the correct stencil is being used. In the photo at right, the usable width is less than 4′.
Guadalupe Road / Tempe Resurfacing 2016
Guadalupe Road is a major arterial running east and west through much of the “East Valley” (Guadalupe, Tempe and Mesa). In the city of Tempe it extends the entire width of the city, and the city takes care of from roughly Hardy Drive to just west of SR101. It has two through lanes and a designated bicycle lane* in each direction, as well as a center turn lane (no raised median) with curb-and-gutter and there is no parking anywhere.
The posted speed limit is 45mph with full-time reductions to 35mph around the two high school campuses. The speed reduction does not “cause” congestion, as I think I hear so often from hi-speed advocates.
Guadalupe Road received a complete resurfacing in the later part of 2016. Very smooth.
The goal after the resurfacing appears to have been to reproduce exactly the striping that was there before.
This was a missed opportunity: a much more “bike lane” friendly pattern could have been chosen, instead of the 4 quite-wide through lanes along with minimal BLs that has (and still is) striped there. There appears to be 68′ curb-to-curb, I would have gone with something like: 2′ gutter + 5′ usable BL / 11′ Lane 2 / 10′ lane 1 / 12′ center lane / 10′ Lane 1 / 11′ Lane 2 / 5′ usable BL + 2′ gutter…. did i get the addition right? I think that totals to 68′. An example of which was done at the Warner Road, also Tempe, resurfacing in late 2016, which seems to have the same 68 feet, and now has 5′ of usable width in the bike lanes mid-block.
I’m going to strikethrough my original statement, just below; as I 311’ed this particular area and spoke with a City of Tempe engineer about it… He measured the area and said that since it’s >= 5.5′, the city “standard”, it’s not an issue from their perspective. He really didn’t or wouldn’t acknowledge even the concept of usable width; repeating only that when measured from the curb it was whatever it was (5.5′ or whatever). He also said, and looking more closely, i now see he is correct, that the stripe is in exactly the same spot as it used to be.
Separately, there’s an obvious mistake — I don’t know if this is a bad measuring job or just what, the existing stripe in the section in the picture, just west of S La Rosa, used to be very close to 4′, perhaps a couple of inches shy… and after the resurfacing is now is unfortunately moved several more inches in a bad way.
This is in an area that has 2′ wide gutter pans — I don’t know if that has anything to do with it or not. (I just mention it because it’s weird, and noone seems to know why they are there. Baseline Road also has segments with 2′ wide pans). ]
* There is a 700′ gap in the BL at S Maple Ave, in order to provide dual left turn pockets at the (offset) intersection with Maple. The treatment of the BL stripe here is improper(?undesirable) — it should not simply gradually bend into the curb, if the BL has to end, it should just end. Likewise the re-start of the BL should not gradually emerge from the curb, it should just start and only when there’s adequate width.