I started a week or two after Hurricane Katrina, when there were reports that NC's governor was going to ration gasoline.
The thing is--and I feel terrible about this--I should have been doing this for 3 or 4 years prior to that. I work in the same building now as I did then and have essentially the same job. I don't know why I didn't start sooner; I even commuted a few times but just never really decided to do it full time.