If you bike to work in Washington, D.C., Minneapolis, Chicago or San Francisco, you're part of a boom. Cycling has at least tripled over the past two decades in these — and other — big cities across the U.S.

"It's almost like a snowball effect," says researcher John Pucher of Rutgers University. "People see other people cycling and they say, 'Wow!' " As part of a three-year research project for the U.S. Department of Transportation, Pucher has completed a preliminary report that documents the increase in biking in nine major North American cities.