Late last week, several media outlets ran stories on a study published in the American Journal of Public Health that allegedly showed head injuries increasing in cities with bike-share programs. Kay Teschke, who studies city cycling at the University of British Columbia, read the news with great interest. Then she read the actual journal publication, and her interest changed to alarm.
"When I actually looked at the data, I thought, oh my goodness, the injuries actually went down," she says. "In the bike-share cities, the total number of injuries went down, and the number of head injuries went down."
Graves and collaborators evaluated injury data in five North American cities (Montreal, Washington, D.C., Minneapolis, Boston, and Miami Beach) 24 months before and 12 months after these places implemented bike-share programs. As a point of comparison, the researchers evaluated injury data over the same time period in five control cities where bike-share didn't yet exist.
Graves and team reported that the proportion of head injuries as a share of total injuries increased in the bike-share cities after the programs began, going from roughly 42 percent to 50 percent. Meanwhile, the proportion of head injuries to total injuries stayed statistically flat in the non-bike-share control cities, going from roughly 38 percent to 36 percent. From that, the researchers concluded that bike-share "is associated with increased odds that a person admitted for a bicycling-related injury would have a head injury."
That's a very nuanced finding, with the key word in the report being proportion. It wasn't that head injuries increased in bike-share cities. It was that head injuries as a proportion of total injuries increased in bike-share cities, particularly in comparison with non-bike-share control cities.
When Teschke reevaluated the core data — found in Table 2 of the AJPH report — she realized the injuries themselves had fallen in bike-share cities. Total injuries per year in these places decreased about 28 percent, and total head injuries decreased about 14 percent, she explained in a tweet last week. By comparison, in the non-bike-share cities, total injuries increased slightly (Teschke tweeted 6 percent, but her figures and those of the study show a 2 percent rise) and head injuries decreased just 4 percent over the same period.
Moderate-to-severe head injuries also seemed to decline slightly in bike-share cities and rise slightly in non-bike-share, though these data were less clear.
So, with that out of the way, now what do we know? Well, first, that head injuries did increase as a proportion of total injuries in bike-share cities after program implementation, and second, that aggregate annual injuries declined in bike-share cities even as they remained about the same in non-bike-share cities. The AJPH paper failed to mention the second and arguably more crucial point. So rather than conclude that bike-share systems might be increasing rider safety, the researchers argued that bike-share systems might improve head safety by offering helmets. In that sense, they seemed to miss the forest for the trees.
"It seems critical to me, especially for people interested in bike-share, to report that injuries overall went down, including head injuries," says Teschke. "That's really important. Especially because it is likely that cycling went up in those cities."
Taken in this new light, the research still poses plenty of intriguing questions. First and foremost is why total and head injuries declined in bike-share cities. The data, which show a correlation rather than a causal link between bike-share use and injuries, can only inspire speculation. One possible reason is that bike-share cities are benefitting from the "safety in numbers" effect; another is that bike-share cities also tend to build better bike infrastructure, which not only enhances safety but encourages riding.
The other question is why head injuries rose as a proportion of total injuries in cities with bike-share programs. It is indeed possible that bike-share users don't wear helmets as often as non-bike-share cyclists, contributing to the relative rise, although if that were the case it's quite strange that head injury severity would decline. It's also possible that bike-share bikes are safer in general — with their heavy frames and wide tires — producing fewer total injuries that require treatment and thus making head injuries appear proportionately more frequent.
Some publications have made an effort to clarify the headlines (the Washington Post now reports that "Proportion of head injuries rises in cities with bike share programs"). The misleading press release headline didn't help matters, nor did the study's silence on the aggregate injury figures. Further confusing matters was the fact that the researchers compared 24 months of pre-bike-share injury data with just 12 months of post-bike-share data, making the study's injury table difficult to evaluate one-to-one at a glance. (Teschke divided the pre-bike-share data by two to get the annual figures.)
At the end of the day, the research on whether or not bike helmets (and bike helmet laws) improve rider safety remains incredibly murky. Meanwhile, the bike-share safety anecdotes are incredibly compelling: Not a single person has died on a Citi Bike in New York. The lesson, as always, is not that helmets aren't safe. It's still always a good idea to be careful when you bike, no matter what city you live in or what type of bike you ride. Just be careful when you read and write about cycling, too.