Usable Width of a Bike Lane
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 below for some engineering details of 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′.
Usable Width of Lanes without Bike Lanes
When on-street parallel parking is provided, the right edge of the usable width is measured to be clear of door zones, generally 11 feet from the curb “NCHRP 766 documents that the open door width of private passenger vehicles extends 11 feet from the curb”, see more here.
On urban roads with a lane adjacent to curb-and-gutter without on-street parking, the right edge of usable space is the seam or joint line between the concrete gutter pan and the (usually asphaltic concrete) road surface.
The left edge of the usable width is the center of whatever lane divider markings are used.
General purpose travel lanes of any width accommodate bicyclists, since they are narrower than any two-track vehicle. The only issue becomes whether or not a given lane is “too narrow to safely share side-by-side with a vehicle”. When a lane is too narrow, the bicyclist FTR (Far to The Right; 28-815A) law is excepted, and bicyclists should ride near the middle of the lane to discourage illegal, too-close passing. AASHTO 2012 describes as follows:
Lane widths that are 14 ft (4.3 m) or greater allow motorists to pass bicyclists without encroaching into the adjacent lane. The usable lane width is normally measured from the center of the edge line to the center of the traffic lane line, or from the longitudinal joint of the gutter pan to the center of the lane line. The gutter should not be
included in the measurement as usable width
However, it must be pointed out the even 14′ is demonstrably not wide enough for general lane sharing (see “zombie myth” here), and can only accommodate sharing with vehicles that are no wider than an SUV or pickup.
Nevertheless, most arterial roads are designed with lanes that are significantly narrower than 14′ of usable width, with 10-12′ being common; and are therefore unsuitable for sharing side-by-side; cyclists should not ride to the right side in such lanes (per expert safety advice) , nor are they legally required to (per Far to The Right law ).
Most, nearly all, urban roads use some form of concrete curb-and-gutter; and not an “open shoulder”.
Seam separation, where the asphalt of the roadway moves away, up, or down, relative to the concrete pan is a persistent problem. [examples: pic at Rio Salado Pkwy at SR101, see pic above of Warner near Rural, see pics of Warner just east of Kyrene] and is one reason why gutter pan width is never counted towards the usable width of a lane, as mentioned above.
Here are some technical details about common gutter pans used in particular in the MAG (Maricopa Assoc of Gov’ts) region, but are very similar to those used elsewhere.
Gutter pans can vary, or not even exist at all. Typically local agencies in MAG uses a 1.5 foot [it’s actually 17″; which is 24 minus 7 on the MAG drawing ] gutter pan next to a vertical curb. (by comparison ADOT typically uses 24 inches). See Vertical Curb and Gutter (Type A), detail 220-1 in the MAG document Uniform Standard Details for Public Works Construction. Each local agency also has their own details that are referred to as Supplement to the MAG.
There are some oddities I’ve only ever seen in Tempe, see e.g. the Broadway Road lane diet project; they use 17″ usual gutters everywhere except it flares to 24″ near each intersection, this squeezes an already shoehorned bike lane even more. Lately it appears on rebuilds, see e.g. Mill Ave between Southern and Broadway, City of Tempe is adding 24″ pans at all bus stops.
It’s also possible, but not recommended, to use a very wide pan as a bike lane; as can be seen a dangerous longitudinal crack large enough to hold a bike tire formed along a portion of Rio Salado Parkway which uses the wide-gutter arrangement.
Besides narrowing the usable width, drainage grates are a particular concern for bicyclists and can cause municipal liability if not designed and properly maintained; see e.g. Fairfield, CT $2.1M settlement in early 2018.
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.
#GutterDoesntCount: A complete guide to why the gutter isn’t the bike lane
There are a bevy of great pics, explanation, videos, and other resources here in this article by Don Kostelec, e.g. here’s one of the videos: