“Cities with bike share programs see rise in cyclist head injuries,” the Washington Post’s erroneous headline stated.
The paper was reporting on a study, published by the American Journal of Public Health, that shows the number of cyclist injuries decreasing in cities with public bike-shares — if you read between the numbers to see this drop.
Bike-shares typically don’t rent helmets along with bikes, so the researchers wanted to see their effect on cyclist head injuries. Tracking the proportion of head injuries to total injuries, they compared data in five cities with public bike-share programs to five cities without over a period of three years. The team studied two years of data from trauma centers before public bike-shares opened and one year of data after they opened. The bike-share cities were Montreal, Washington, D.C., Minneapolis, Boston and Miami Beach while control cities included Vancouver, New York, Milwaukee, Seattle and Los Angeles.
In bike-share cities, the paper states, “the proportion of head injuries among bicycle-related injuries increased.” Researchers reported an uptick from 42.3 percent before the public services were implemented to 50.1 percent afterward. Meanwhile, in cities without bike-shares the proportion dropped slightly, from 38.2 percent to 35.9 percent over the same period of time.
“Results suggest that steps should be taken to make helmets available with PBSPs,” the paper concludes. “Helmet availability should be incorporated into PBSP planning and funding, not considered an afterthought following implementation.”
But because the study looks only at proportion, its findings are easy to misinterpret. Hence the Post headline, which has since been corrected. The number of total injuries, including head injuries, in the bike-share cities actually fell after programs were implemented.
One of the first people to point this out was Kay Teschke, a University of British Columbia public health professor and lead investigator in the Cycling in Cities Research Program. In a comment on the article and later in a letter sent to the journal, she noted that this trend — that all injuries actually went down in bike-share cities — wasn’t clearly outlined in the paper.
Teschke’s conclusion came from the paper’s injury data (see the table below). As it was laid out, the numbers seemed to suggest that all injuries went down from pre- to post- implementation of bike-shares, but Teschke realized this was partially because the pre-implementation phase had actually lasted two years, while the post-implementation phase lasted only one. She did some calculations and realized that the numbers actually did show all injuries, including head injuries, declining.
Injuries in cities that had implemented bike-shares, according to the table, dropped from 757 to 545 per year, a decrease of about 28 percent. Meanwhile head injuries also dropped, from 319 to 273 per year, a decrease of about 14 percent. Control cities saw a slight uptick in injuries over the same period, from 932 to 953 per year, and a slight drop in head injuries, from 356 to 342 per year.
The study’s lead researcher, Janessa Graves of Washington State University’s College of Nursing, said in an email that these numbers don’t tell the whole story, which is why the researchers focused on proportion.
“Evaluating crude numbers alone, without considering the underlying population or denominator (e.g. number of riders in each city), is not entirely appropriate, even when we assume ridership increased,” she writes. “We did not have those numbers for this study, so could not evaluate the NUMBER or RATE [emphasis hers] of injuries. That is why we looked at proportions and risk.”
Graves adds that because her team did not know whether ridership increased, decreased or stayed the same in cities with bike-shares, they were reluctant to extrapolate. The total number of injuries may have gone down, but what of the total biker population?
Some critics of the study, she states “assume that the number of cyclists increased in bike-share cities and likely stayed the same in non-bike-share cities, however, we do not know this for certain. That is why we could not look at this outcome.”
But bike-shares and lanes do tend to result in an increase of bikers overall. Teschke, who has also been published by the AJPC for her research on bike infrastructure, feels that although the study looked at proportion it didn’t really assess risk.
“If you go out cycling, what is the risk of a head injury?” she asks, adding that in places where there’s more cycling overall, cyclists’ risk of injury tends to go down. Possible reasons include infrastructure and the fact that cars pay more attention to cyclists because more drivers also bike.
Regardless, Graves and her team highlighted some interesting trends regarding head injury and helmet use. Shares would be wise to implement policies based on the higher proportion of brain injuries reported. But it’s also possible that, simple by existing, they’re actually making cyclists safer too.
The Works is made possible with the support of the Surdna Foundation.
Rachel Dovey is an award-winning freelance writer and former USC Annenberg fellow living at the northern tip of California’s Bay Area. She writes about infrastructure, water and climate change and has been published by Bust, Wired, Paste, SF Weekly, the East Bay Express and the North Bay Bohemian