San Francisco has a very good public transit system, at least compared to any other suburb in which I've lived. In San Francisco, I could be most places I cared to be within 30 minutes. There was literally a bus station outside of my apartment window, and I could catch probably a dozen different bus lines within a six block radius, maybe more. I lived without a car for over 5 years in SF, and it was fine.
Alexandria, VA is different. The closest Metro Station a light rail system, is a 20 minute walk by foot, and it has little to no parking. Pretty much anything I want to do is at least a 20 minute walk away. At the moment I can borrow my dad's car, but if I had to walk, my life would be dominated just by trying to get around. Sure, I can survive without a car in Alexandria, VA, but it'd kind of a pain in the ass.
I can't imagine trying to live in a rural area without a car.