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Defining Narrow Width Lanes

NOTE: This post was updated on 9/11/2013.

Here in Minnesota, cyclists are legally permitted to “take the lane” as specified in Statute 169.222 Subd. 4, which says:

Every person operating a bicycle upon a roadway shall ride as close as practicable to the right-hand curb or edge of the roadway except […] when reasonably necessary to avoid conditions, including […] narrow width lanes, that make it unsafe to continue along the right-hand curb or edge.

So how do we define a narrow width lane? The statutes don’t define a narrow width lane, and Mn/DOT has never formally provided a definition of this term either.

Several documents provide some guidance about how we might apply the “narrow width lane” statute, including some conflicting guidance on whether the gutter pan should be included in the lane width measurement:

  • The 2005 Mn/DOT Bicycle Modal Plan states that it may be appropriate to use signage indicating that cyclists may use a full lane “on urban streets too narrow (3.9 meters [13 feet] or less) for bicycles and cars to safely pass within the lane.” (page 70)
  • The 2012 AASHTO Guide for the Development of Bicycle Facilities states “Lane widths of 14 ft (4.3m) or greater allow motorists to pass bicyclists without encroaching into the adjacent lane.” (page 4-3)
  • The 2007 Mn/DOT Bicycle Facility Design Manual states “For accommodating bicyclists, the wide outside lane dimension should not include the gutter pan,” (page 90) and “Most practitioners agree that on urban streets without parking, the minimum space necessary to allow a bicyclist and motorist to share the same lane is 4.2 meters (14 feet), measured from the lane stripe to the edge of the gutter pan, rather than to the curb face.” (page 92) This applies to roadways with a 35 mile-per-hour speed limit (or less) and without on-street parking. As the speed limit increases or parking is allowed, the minimum width recommended to allow bikes and cars to share a single lane also increases.
  • The 2012 AASHTO Guide for the Development of Bicycle Facilities states that “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 lane line. The gutter should not be included in the measurement of usable width, as bicyclists will typically ride well to the left of the joint.”
  • MnDOT State-Aid Rules state that “The dimensions for wide outside lanes include the curb reaction distance,” which typically includes the gutter pan. These rules have limited application tied to certain roadways and funding sources. However, they are often used as general guidance as well.

So how do these dimensions compare to typical roadway lane widths?

  • On new construction, standard lanes are 12 feet, but 11 feet is very quickly becoming the new standard. State-Aid standards (or other standards) usually require a minimum shoulder width of 2 – 4 feet, some portion of which will be gutter pan. Assume an agency is using 11 foot lanes and 1.5 foot gutter pan. If the gutter is not included in the usable roadway space, the effective lane width on a new construction roadway will typically be 11.5 – 13.5 feet. If the gutter is included in the usable roadway space, the effective lane width on a new construction roadway will typically be 13 – 15 feet.
  • On older roadways it is common to find lanes anywhere from 9-12 feet wide, and there may or may not be any shoulder or “reaction distance” enlarging the effective lane width.
  • It is unclear how the presence or absense of a “fog line” (the solid white line that separates the right-most lane and the shoulder) influences how the outside lane is measured. Depending on circumstances, engineers may or may not decide to stripe this line. See here for further discussion of fog lines and curb reaction zones.

Given this guidance, my impression is that within the Twin Cities central cities area, there are relatively few locations where the effective lane width is 14 feet or greater, especially if gutter pan is not included in this dimension, suggesting that cyclists should be “taking the lane” frequently. This is consistent with my own personal riding habits.

What do you think of the guidance provided my Mn/DOT and AASHTO? Do you think 14 feet is a reasonable minimum dimension? Or do you have no idea since you probably don’t carry a tape measure along with you when you ride?

Thanks to Steve Clark at TLC for drawing my attention to the Mn/DOT guidance on this subject.

2 comments to Defining Narrow Width Lanes

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