That's why I think everyone should bike to work if possible. It has little to no environmental impact and a positive health impact. And because you are killing two birds with one stone by exercising during your commute, you really aren't wasting any time.
I have yet to meet anyone who has communized by bike in my city (Vancouver BC) for more than a few years without a major altercation with a motorvehicle. It might be healthy, but it is also physically dangerous.