Does this or any other named shoulder (a.k.a. bike lane) compel motorists to go slower?

Among other alleged benefits, bike lane proponents often claim that narrowing a wide lane with bike lane striping results in slower motor vehicle traffic. This is supposedly due to visual narrowing of the roadway which theoretically “tricks” motorists into slower speed, or else the physically narrower space constrains speed.

Do Bike Lane Stripes Calm Motor Traffic? reviews 2 publications that have examined the application of edgelines intended to narrow the travel lane in order to provide a traffic calming effect.

6 Responses to “Do Bike Lanes Calm Traffic?”

  1. The graph from the Portland report is a line graph when it should be a bar graph. It does not show a progression, but rather, only a comparison.

    I would like to suggest that under some conditions, narrow lanes can slow traffic — when they actually delineate limited space rather than only try to create a perception of it. I recently had the displeasure of driving on a hilly, curvy 4-lane arterial (the Jamaicaway in Boston, Massachusetts), 44 feet wide, striped with 2 lanes in each direction, with heavy but uncongested traffic. It was necessary to hold speed to 30 mph or less to reduce the risk of collisions. But the experience wasn’t one of traffic calming, it was one of constantly being on edge because of the minimal lateral clearances.

  2. Phillip Troutman Says:

    I don’t think you’re considering what “significant” change really means:

    “It was done in an area in which 85th percentile speeds
    initially decreased at 4 cross street sites from 30-36 mph to 24-31 mph due to the installation of
    speed humps and chokers (Figure 5.1). The N. Ida corridor was obviously already a low speed
    roadway, and certainly not a location to warrant the micro-regulation of bicycle drivers with a
    bike lane.”

    There is a big difference in the experience of being passed by a car going 24 mph vs. 36 mph. From a reaction/stopping time point of view, it makes a huge difference. At, say 60 feet away, a car going 24mph will have time to stop (24 mph = about 54 feet/sec.) but a car going 36 will not (36mph = about 81 feet/sec.).

    “After bike lane installation, speed at three of the four study sites was reported to further decrease
    by 2 mph, and at 1 site by 5 mph, an average of 2.75 mph. It is doubtful that these decreases in
    speed are statistically significant.”

    As averages, these are actually quite significant, especially given that the speeds we are talking about are in the 20-40 mph range (a 2.75 mph average reduction in a 55 mph zone where people were really going 65 would indeed be a less significant reduction).

    Also note that “As
    shown in figure 5.1, 85th percentile speeds (the speeds below which 85 percent of
    vehicles travel) declined by 4 to 7 mph at four points along the project with first
    phase improvements, and by another 2 to 5 mph in the second phase. This brought
    85th percentile speeds down to the speed limit of 25 mph at certain locations, and
    close to it at others.”

    Isn’t it significant that speeds were brought in line with the actual speed limit? I’d say this was pretty effective.

    I lived in Charlottesville, Va., when they started striping bike paths on roads already wide enough for them. As a driver, cyclist, and a pedestrian, I noticed a significant reduction in people’s speeds. (Have you ever tried to turn onto a busy main road–in a car–when people on the main road were driving 35 instead of 25? It can be quite difficult.)

    Finally, many traffic calming use of bike lanes happens in areas that are neighborhoods or commercial areas, where you have a lot of pedestrian traffic. Again, drops in speed limits of even these “small” numbers is quite significant in terms of visibility and safety.

    • Wayne Pein Says:


      The initial changes at N. Ida were from 30 to 24 (6 mph difference) and 36 to 31 (5 mph difference) due to speed humps and chokers, not bike lanes. Your comparison of 24 vs 36 doesn’t exist.

      I’m sticking to my assertion that a 2.75 mph average decrease is not only not statistically significant but also not practically significant. Further, the Portland study was not scientifically rigorous so can’t be trusted. They didn’t prove that bike lanes lowered speed. I believe that the ITE and the Raleigh study that I reviewed are better sources of information.

  3. […] lane on an arterial roadway will reduce motor vehicle speeds on that roadway. This is not true. Here is a link to an article with data from the Institute of Traffic Engineers reviewing this idea, as well as […]

  4. Daniel Keough, City & Regional Planning student Cornell '16 Says:

    I didn’t see any information about whether much of the motor vehicle traffic was making use of the bike lanes, e.g. on curves to the right, the tendency is for the driver to cut the corner, encroaching onto the bike lane. BUT if there was a study with protected bike lanes, or lanes where the areas would have otherwise be worn receive some sort of protection
    such as:
    then I am wondering if the results would be any different.
    If cars are traveling 30 mph and drivers are allowed to make the bike lane an extension of the main travel lane, they won’t need to slow down for the inside curve.

  5. Wayne Pein Says:


    Motorists can be observed doing the 50 mph speed limit in narrow 10 ft lanes with no shoulders on rural roads in NC. So it seems unlikely that at 30 mph their encroachment into a bike lane would be to maintain speed.

    Those rumble bumps create a diversion fall hazard for bicyclists, and prevent left turns from the bike lane.

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