Curb Lane Widths don't Include the Gutter

This is a bit of a public service announcement as it clears up some confusion between lane width dimensions stated in various design guidelines. In summary, as the title says, curb lane widths don't include the gutter. Many progressive and well respected guidelines do not define exactly what does and does not constitute curb lane width. While narrower lanes are encouraged to increase driver discomfort and slow down vehicles, lanes that are too narrow can lead to issues with large vehicles either striking mirrors or having to take evasive action to avoid such a strike.

The Transportation Association of Canada (TAC) Geometric Design Guide for Canadian Roads explicitly states that the gutter is not included in the lane width. That's clear, but many other guidelines quote similar widths without making that distinction.

A frequently used and well respected resource is the National Association of City Transportation Officials (NACTO) Design Guides which are unclear on the subject. They simply provide a lane width and their sketches appear to show that width to the face of curb (below left).  I reached out to ask if the lane widths they suggest include the gutter and found that they do not (see tweet below right).

TAC suggests 3.3m lanes plus the gutter width for lanes carrying transit. Gutters are typically 0.3m to 0.4m, which would typically result in a lane of 3.6 to 3.7m from face of curb to edge of lane. This seemed wildly excessive compared with NACTO. Based on the statement above, it appears these two commonly used sources are broadly in line with each other with NACTO actually being a little more conservative given 11ft is 3.35m, and therefore 0.05m wider than the TAC minimum. Some cities such as Surrey are adopting narrower 0.15m gutters which helps to reduce the overall cross-section width.

I was curious if this played out in reality. I took a measuring wheel and went to measure some curb lanes with buses in them to see if 3.3m plus the gutter seems excessive or about right. I started on Hastings Street in Burnaby. Here the curb lane is 3.0m with a 0.4m gutter. the bus is 2.6m wide or about 3.2m with mirrors. Below, with the bus stopped partially in the gutter the left mirror is close to the lane line. When the bus is travelling, typically in the centre of the lane, its left mirror would likely push into the adjacent lane.

I walked around the corner to Willingdon Avenue. Here the curb lane is 3.4m while the gutter is 0.4m, 3.8m in total, the bus wasn't stopping and as I was standing at the curb he moved over a little, still staying mostly within his lane with a fair bit of room to spare.

What is the sweetspot? I think a key criteria should be a for a vehicle to drive along the centre of its lane without intruding on the adjacent lane. If a bus is 3.2m mirror to mirror, this seems like a reasonable starting point for a lane carrying buses. The 3.3m in TAC and 3.35m in NACTO offer a small buffer which also seems reasonable.

Further to this, I also had some discussion with BC Transit who also have a design guideline with 3.3m recommendation for lane widths for buses. While their guide doesn't comment on the inclusion or not of the gutter in that width, they confirmed it is not included, and that they align with the TAC guidance.

The new BC Active Transportation Design Guidelines, doesn't focus too much on vehicle lane width, but provides some interesting commentary on the subject. With regard to the Ancillary Zone it states "The Ancillary Zone is a flexible space located on-street within the roadway that is not designated for motor vehicle through traffic, but that supports the primary functions of either the roadway or the sidewalk. Uses can include on-street motor vehicle or bicycle parking, bicycle facilities, docked bike share stands, loading zones, transit stops, taxi or ride hailing zones, curb extensions, parklets, or patios. This space also includes the concrete gutter and, depending on 
the road design, may be used for snow storage." It also frequently states that "gutters are provided for drainage" i.e., not for use as a travel lane.