“Ok, so if you ride your bike regularly in place of driving a car you might think you’re doing the environment a favor. Well, according to a Wharton School professor it’s time to shelve that holier-than-thou attitude. Based upon Karl Ulrich’s research (.pdf paper), biking does save a considerable amount of energy. But it also makes you healthier. Ulrich finds that a single additional year of biking adds 10.6 days to one’s life, and that the energy consumed during this added lifetime offsets the energy saved by pedaling” (http://blogs.chron.com/sciguy/archives/2006/09/go_bicycling_de.html).
And don’t forget to wear a helmet!