This is a great topic, and one that cyclists think of often. I'm no saint: my wife and I own two vehicles, Subaru Forester and VW Eurovan camper. I figure we drive a total of 10 to 15k miles/year--pretty average N.American drivers, I suppose. Still, we do a lot with our bikes and on foot even though we live in a hilly, mountainous area. Being in the southern Sierras of California, we are blessed with a lot of good weather, so even in the winter we can ride some.
I live (gasp!) 45 mi. from my place of work; however, I take public transportation 95% of the time, almost always in conjunction with my commuter bike. I ride the 3 mi. each way (hills both directions) to the bus stop and then ride the bus. This little ride is often the highlight of my day. With a RARE exception, I am the only one to ride to the bus stop. The bus goes right to the college where I work, and many of the riders are young students who should be fit enough to ride to the bus as I do, yet none of them ever does. As I walk about my campus (Bakersfield California), I realize that FAT seems to be the norm. I am the freak.
Granted, given the current state of US cities and roadways, full-time biking can be a serious challenge, but we can EASILY to a lot more than we currently do. And here's the secret that the sedentary don't understand: biking is fun! Loading up the panniers or the trailer for a grocery run is just a great way to spend some time. I never regret a human-powered outing, never.
Anyway, enough rambling.
Cheers,
Scott