Arterial lane width is a topic that comes up a lot lately as cities are moving toward shrinking car lanes and adding bike lanes, sidewalks, or bumpouts on dangerous arterial roads. As that’s been happening in Saint Paul, I’ve encountered many people questioning whether 11′  lanes are safe.

“How can buses pass each other?” or “I went out and measured the street…”, people ask.

But we have a lot of data on lane width and safety. Here’s a chart from the National Association of City Transportation Officials (NACTO):

The NACTO Street Design Guide explains the relationship here:

The relationships between lane widths and vehicle speed is complicated by many factors, including time of day, the amount of traffic present, and even the age of the driver. Narrower streets help promote slower driving speeds which, in turn, reduce the severity of crashes. Narrower streets have other benefits as well, including reduced crossing distances, shorter signal cycles, less stormwater, and less construction material to build.

Of course, it’s always more complicated. There are a few caveats:

For multi-lane roadways where transit or freight vehicles are present and require a wider travel lane, the wider lane should be the outside lane (curbside or next to parking). Inside lanes should continue to be designed at the minimum possible width. Major truck or transit routes through urban areas may require the use of wider lane widths.

Lane widths of 10 feet are appropriate in urban areas and have a positive impact on a street’s safety without impacting traffic operations. For designated truck or transit routes, one travel lane of 11 feet may be used in each direction. In select cases, narrower travel lanes (9–9.5 feet) can be effective as through lanes in conjunction with a turn lane.²

In general, narrower lanes on urban arterials are safer than wider 12′ (freeway-standard) lanes.

Does anyone know what Mn-DOT’s state aid standards say about lane width, and whether these kinds of conversations are changing?