The streets of big cities such as New York, Chicago and San Francisco have gotten a little more crowded lately — not with automobiles but with bicycles.
Since spring, all three have launched bike-sharing programs, joining a raft of other cities in the United States and around the world that allow people to rent bikes by the hour or day.
Thanks to the newcomers, the U.S. bike-sharing fleet has doubled since the beginning of 2013, and it is expected to double again by the end of 2014, according to the Earth Policy Institute.
Public-health officials see just one problem with this: bike sharers, for the most part, are not wearing helmets when they ride. And they’re worried how many riders will show up in emergency rooms.