Study: Sharrows Don’t Appear to Increase Bike Safety


Will sharrows reduce the chances of getting doored? The best place to look, one study found, is in Chicago, where the city actually examined the extent of dooring in 2010. The study was just accepted for publication this year, though.

One reason for sharrows is to encourage cyclists to move out of dooring range. The researchers compared crash data on Chicago streets with sharrows to those on streets with marked bike lanes and those with nothing special for bikes.

Sharrows didn’t seem to help much. The authors realize they couldn’t determine why, but found areas “that had sharrows installed experienced less than desirable safety outcomes….” They couldn’t say why for sure but suggest that sharrows may “provide a false sense of security to bicyclists” since bike lanes provide dedicated space and cyclists know to look out if there’s no marking. Sharrows may also attract inexperienced cyclists.

See N.N. Ferenchak, W.E. Marshall, Advancing Healthy Cities Through Safer Cycling: An Examination of Shared Lane Markings, International Journal of Transportation Science and Technology (2019), doi:


(Visited 370 times, 1 visits today)


  1. I’ve read the study cited above, although it was a difficult exercise. Why? Because the “nonsense” frequency gave me a headache from all the eye-rolling I did!

    Example: The authors begin by claiming the only initial motivation for sharrows was to reduce dooring. That’s absolutely false. They then claim that dooring is a rare problem, despite saying that in Boston, doorings were 11% of car-bike crashes, and 29% of bike messenger-car crashes. Other sources say 20% of Chicago bike crashes are doorings. Those numbers don’t qualify as “relatively rare and benign.” And the author’s polemic question, “Why would we install sharrows that avoid doorings at the cost of of increasing risk for vehicle collisions?” is nonsense, because it displays the author’s hidden assumption that riding more prominently in the lane increases risk of collisions. Most truly competent cyclists have found the opposite to be true, and _that_ is a major point of sharrows!

    Another example: The authors then suddenly switch to claiming that the intent of sharrows is (instead?) to increase passing clearance by cars. They claim the results are “highly variable,” but cite several studies in which that spacing was, indeed, found to increase, by anything from several inches up to 2 feet. They also mention studies correlating sharrows with less sidewalk riding, greatest benefit on four-lane roads, etc. Yet they disparage sharrows as ineffective.

    It’s significant that the paper does not ever count actual cyclists using actual sharrows! (Nor bike lanes, nor plain streets, for that matter.) Instead, it uses approximate data on bike _commuting_ as a proxy for total ridership. And it generates its risk ratios by using _all_ cyclist injuries. In other words, a wrong-way midnight drunk’s crash is divided by the number who claim in surveys to be commuters.

    And not the number of commuters on the street where the drunk crashed! The authors use approximate data on city “block areas” of unspecified size. If a “block area” has some streets with nothing special and one street with a short section of sharrows, it apparently counts as a sharrow “block area.” Furthermore, the before-after computations that purported to evaluate the cycling treatments don’t use even the same “block areas.” Those boundaries shifted over the study period, so the areas are only approximately similar.

    Given all those shortcomings: What the authors actually found was that “block areas” with sharrows saw an increase in bike mode share (based on surveys, not on observation) from 0.69% to 1.32%. As with “block areas” with no changes (0.33% to 0.75%) or those with bike lanes OR recreational bike paths (0.64% to 1.78%), those changes might best be characterized as “from negligible to negligible.” However, all are increases.

    Likewise, the injuries (of _all_ cyclists) per 100 bicycle commuters dropped from 33.3 to 21.1 for no treatment, from 31.2 to 25.1 for sharrow-containing “block areas,” and from 59.2 to 34.4 for bike lane “block areas.”

    Now think about those magnitudes. 59.2 reported injuries for every 100 bike commuters! How can that be? Again, its’ _all_ reported bike injuries – the drunks, the kids falling off their bikes and rushed to ER by helicopter parents, the wrong-way-sidewalk riders, the stunt riders and more. It makes sense only because the commuters are a negligible portion of the total – yet that negligible portion is used as a denominator in evaluating safety. The logic is mathematically weird.

    Note that the sharrow “block areas” did experience an increase in cycle commuting, as well as a decrease in (computed) crash rates. How, then, do the authors justify their statement that “sharrows have less than desirable outcomes”?

    The authors do a fair job of listing the shortcomings of their methodology: the assumption that the impact of bike infrastructure would be experienced _thoughout_ a “block area” if any street gets some bike facility; the assumption that bike commuter survey responses accurately indicate _total_ bike use. They admit that the proper way to evaluate both ridership effects and safety effects would be by accurate before-after counts, but say they chose their strange methods because – well, because it was easier to get this data!

    But they also hint at perhaps the biggest shortcoming: the implied direction of causality. The entire paper implies that the bike lanes _caused_ more increase in cycling than the sharrows, and _caused_ greater increases in safety. Segregation-promoting websites like Streetsblog have certainly taken up that cry. However, four paragraphs before the end, they say “the results of this research do not imply causality…”

    Right. In effect, its an admission that their paper’s message is not justified by their own data, as weak as that data is.

    To explain: There could be many undetected or ignored confounding factors behind the data. Did bike lanes increase cycling more where they were installed? Or were the bike lanes installed because those “block areas” had lots of cycling, good cycling conditions and lots of commuting potential via short trip lengths, hip local culture, accommodating businesses, etc.? Did those already cycling, and right on the edge of commuting, demand the bike lanes? Would the commuting have increased just as much during this period of growing bike fashion? We don’t know; but it would not be at all surprising if DOTs installed bike lanes where cycling was growing the fastest and requested the most – in other words, where cycling was already pretty good.

    Finally, the authors tip their hand in their final three paragraphs. These young research assistants in Denver constructed this paper because, according to them, Denver is on track to completing its planned sharrow installations, but it is “falling behind” on other facilities. (Yes, gentlemen, installing any bike lane, especially a “protected” one, is more time consuming because of the need for acquiring and clearing right-of-way, or studying the effects of reducing traffic lanes. Separate bike trails are even more complicated. Who could be surprised that sharrow installation happens more quickly?)

    Here’s their final polemic: “It is time that sharrows are exposed for what they really are, a cheap alternative that not only fails to solve a pressing safety issue, but actually makes the issue worse through a sense of false security.”

    That’s what I call a “Danger! Danger!” argument. “Oooh, riding a bike is SO risky that it’s a pressing safety problem!”

    Let’s keep in mind that data shows Americans ride over 10 million miles per year between fatalities. And that, per mile traveled, those walking experience three times as many fatalities as those riding bikes. Those numbers, BTW, come from John Pucher, who – despite the data’s message – is another “Danger! Danger!” proclaimer regarding bicycling.

    It is flatly anti-cycling to exaggerate the danger of bicycling, and to claim that one dare not cycle until one has completely segregated infrastructure. It suppresses cycling here and now, and tends to blame those few victims who are injured while riding legally, using their rights to the road. “Advocates” of that stripe need to be exposed for the fear mongers they are.

    Meanwhile, the main benefits of this paper are exposing those who buy its message. They place themselves firmly in the fear-mongering camp, and illustrate their gullibility and their bias.


Please enter your comment!
Please enter your name here