You know that bumper sticker on your Prius that says "My Other Car is a Bike"? You might want to slap on another next to it that says "And It's Not Even My Bike," because bikesharing systems are totally hot right now.

There are now 400 bikeshares in western Europe, up from "a few" 10 years ago. The world's biggest bikeshare — in Hangzhou, China — has 50,000 bikes available and logs 240,000 trips a day. Arbiter of cool New York City has plans to launch a bikeshare system by April, 2012. There are bikeshares across five continents — Mexico, Brazil, Canada, South Korea, Taiwan, Australia, and Israel all have them. Basically, bikeshares are bigger than the iPad 2.

Not all the programs are as successful as Hangzhou's; the Melbourne bikeshare service has been falling short of expected ridership, for instance. They don't work as well in cities without good biking infrastructure, or with political opposition to biking. But they're cheap to start and cheap to operate, and they cut pollution, lower gas expenses, reduce traffic, free up space previously occupied by parking, and make people feel more in tune with their cities — any two or three of which could explain their booming popularity.

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