Chart of the Day: Lane Width vs. Speed on Suburban Streets

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):

wider-travel-lanes-graph

 

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?


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5 Responses to Chart of the Day: Lane Width vs. Speed on Suburban Streets

  1. Adam Froehlig
    Adam Froehlig November 4, 2015 at 6:46 am #

    A recent study on urban lane widths versus crash rates found 10.5ft to be the sweet spot instead of 10 or 11. Crash rates start increasing again when going less than 10ft, so I would rule those out.

    Regarding your question, there are 4 different categories of roadway design standards. Categories depend on whether on-street bike facilities are included or not, or whether it’s a reconstruction or reconditioning. Reconditioning is defined here in layman’s terms as a pavement resurfacing…it can include minor curb improvements, but along no more than 20% of the project length.

    Urban reconstruction (or new roadway) projects.

    Urban reconditioning projects.

    Urban new/reconstruction with bike facility.

    Urban reconditioning with bike facility.

    Generally speaking, lane widths of 10ft can be selected *IF* the following criteria are met:

    – Traffic volumes less than 10K.
    – Speed limit is 30 or less.
    – The street/road has a bike lane.
    – Bike lane and parking lane widths are at least 1ft wider than minimum (so 6ft for bike lane and 8ft for parking lane).

    • Alex Cecchini
      Alex Cecchini November 4, 2015 at 9:00 am #

      That’s a pretty narrow reading of the findings from the recent study. They actually gave a “safest” band of 3 to 3.25m, or 9.8′ to 10.6′. Additionally, this only considers traffic crash rates, not severity. Particularly for non passengers.

      They note that roughly 45% of Tokyo’s streets have lane widths less than 3m (9.8′) compared to Toronto’s ~15%, yet they have much higher traffic volumes on average and 7x the population. They also found that speeds were 34% higher in Toronto collisions than Tokyo’s. So if we care about total deaths and severe injuries, that shifts the sweet spot down a bit.

      Th again find that “Contrary to common belief, the results clearly demonstrate that narrower travel lanes, particularly 3.0m lanes, carry the highest traffic volumes (18% higher compared to 3.5m lane)” using Toronto’s data. Despite narrower lanes on the whole, heavy truck traffic made up 15% of all vehicles entering intersections vs 4.9% in Toronto. This may say as much about mode choice as it does anything else, but they go on to note that 3m lane widths don’t pose any congestion risks to truck traffic based on their data.

      We can, of course, argue over priorities of safety, commerce, etc and how that should play out for design on different streets, but I see no reason a 10′ lane design, particularly on streets with multiple lanes, shouldn’t be the standard.

  2. David Markle
    David Markle November 4, 2015 at 11:15 am #

    No big surprises in this chart, and Froehlig and Cecchini follow up with good points. It would be good to see a chart that includes crash rates and traffic (usage) rates as well, and charts that compare these figures before and after the narrowing of automotive lanes to accommodate bike lanes.

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