Chart of the Day: Lane Width vs. Crash Severity

Here’s a fascinating chart making the rounds today, from a meta-study looking at how lane width impacts speed and (thus) crashes, injuries, and safety on streets:


The trough of the curve (the place with the least severe crashes) seems to be right between 10.5′ and 11′. That’s something that cities should keep in mind, because the default assumption in many planning conversations is that wider lanes are saver (see also: forgiveness). As it turns out, some research disputes that notion, especially in urban areas.

The report goes on to mention the potential uses of that extra couple of feet you might get from narrowing travel lanes. You could increase room for bikes, parking, pedestrians, or transit, all while making the road safer. Seems like a ‘win-win.’


12 thoughts on “Chart of the Day: Lane Width vs. Crash Severity

  1. Walker AngellWalker Angell

    Great chart and not too surprising. Roads throughout Britain are quite narrow and yet they have one of the safest road systems in the world. Northern Europe is very similar. If narrow lanes were so dangerous why are their roads so much safer? If wider lanes are so much better why are our roads so much more dangerous?

    On Shelby Avenue between Dale and Western in St Paul cars drive fairly fast on the Western end but much slower and more carefully on the Dale end where it is much narrower.

    While lower speeds is part of the equation I wonder if narrower lanes force people to pay much better attention and that is the more critical element of their safety. Perhaps Dangerous is Good.

  2. Monte Castleman

    Does this study apply to freeways, where you can have up to 85 mph speed limits and lots of heavy trucks? But it’s interesting that lanes that are too wide can be dangerous at high speeds because smaller cars will drift from side to side in the wide lanes. I believe 13-14 foot lanes are where the problems start, and above standard width lanes are problematic in the cities too, because two cars will try to squeeze side by side.

    1. Matty LangMatty Lang

      I don’t know the answer to your question, but aren’t the I-94 lanes in Minneapolis quite narrow (for freeways) since MnDOT permanently slipped in an additional lane that was supposed to be temporary until the 35W bridge was replaced?

      1. Monte Castleman

        Froggie might know if it I’m wrong, but I believe that they’re 11 foot lanes, which while substandard are kind of OK on freeways in certain situations, even for new construction like the I-35W access project.

        1. Adam FroehligAdam Froehlig

          MnDOT has occasionally used 10.8ft lanes where right-of-way was constrained. They’ve gone as low as 10.5ft in construction zones. But I believe you’re correct in that the I-94 lanes are 11ft.

          The proposed 35W/Lake Access project will have 11ft lanes on 35W between 42nd and Lake St. North of Lake will be the normal 12ft.

  3. Alex CecchiniAlex Cecchini

    Just a point of clarification, you state “The trough of the curve (the place with the least severe crashes) seems to be right between 10.5′ and 11′.”

    But that’s where crashes are least prevalent, not severe. I’ve seen mixed research on if sub-10′ lanes further reduce speeds significantly (relative to other design elements that might calm traffic), but it’s possible that 9.5′ lanes might be safer despite a higher crash rate, especially when considering non-motorists.

      1. Alex CecchiniAlex Cecchini

        So 9′ lanes for non-thru streets, 10.5′ lanes for 2-way thru-streets, 10′ lanes with a center turn lane or multiple lanes in each direction. Seems simple enough.

        1. Monte Castleman

          FHWA guidelines are actually OK with 9 foot lanes for local streets (where there’s presumably only a truck every now and then). Collectors should be at least 10 feet, and arterials 11 feet (which Hennepin County seems to be adopting on their newer designs rather than 12). I’d be find with these in urban and inner ring suburban areas where land is at a premium and there might not be space to build standard width lanes, or the space might be better used for other things.

      2. GlowBoy

        The answer is that when two such large vehicles meet, they take it slowly and carefully. As a vehicle that size should be doing in an urban area anyway.

        At least in Portland, after a cyclist was killed by a semi making a turn downtown last year, there’s been serious discussion about whether semi-trucks should be banned from urban streets altogether. As is already the case in many European cities. Most 53′ trucks making deliveries in the city are not long haul: they’ve come in from a distribution center on the fringe of town anyway.

    1. Dewan Karim

      Narrower lanes (like 9ft) is suitable for left-turn lanes or low-volume streets like busy retail areas, residential collectors and local streets. The 11ft is applicable when there is significant volume of large vehicle of buses (in my research it was identified more 5~6% of large vehicle volume). With buffered bike, there is no need to apply 11ft, 10ft is still good enough buses or trucks next to buffered bike lane.

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