As Chicago and New York jump on board, bike-sharing programs hit streets in largest US cities

The newest piece of Chicago's transit puzzle will not rumble between skyscrapers on its 100-year-old elevated rail lines or utilize its fleet of hybrid buses. The city is turning to thousands of bicycles to send commuters and visitors zipping on their way.

Chicago joins New York, Los Angeles and San Francisco this year in catapulting what was once just an eco-friendly option for smaller cities into the transportation mainstream of the largest urban expanses.

The concept is simple: Grab a bike from a curbside station, ride for up to 30 minutes, then lock the bike at any docking point.

Chicago's program, named Divvy, launches June 28 with about 750 bikes at 75 solar-powered stations. It will expand over the next year to at least 4,000 bikes at 400 stations.