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.
Unfortunately, where I'm at, four or five months out of the year, it's below freezing and even if you could brave the cold, you'd be taking your life in your hands with the narrowed roads, drivers that cannot navigate snowy/icy conditions, and snowplows.
Also, fortunately, few people live in such extreme climates as us.