"All of a sudden, you've got these 7,000 bikes, which anyone with a credit card can use. ... My guess is the people using those bikes are far less likely to be experienced cyclists," he says of New York's new system. "What I fear is you're going to have indeed a spiking--and it could be a doubling or a tripling--of injuries and fatalities, both of cyclists and pedestrians."
While the study may show that bikeshare users aren't wearing helmets, it doesn't probe riders' motives for not doing so. It may be due to convenience, as Pucher says, but one of the study's authors says that there may be parallels in other types of transit.
"There's interesting research that people who jump into taxis are less likely to wear a seat belt than if they're driving their own car," says Kraemer. Why that may be is itself a mystery, he says, but understanding why is on his docket. "That's the next step of what we're hoping to do."
Danielle Kurtzleben is a business and economics reporter for U.S. News & World Report. Connect with her on Twitter at @titonka or via E-mail at firstname.lastname@example.org.