I would argue they don't rely on it so much as make use of it. As cities grow then they need to truck supplies in from elsewhere, but for suburbs and rural areas they can survive on the local economy.
Maybe. It depends where you live, though--not every area is capable of sustaining its own food supply(1). And, even those that are still import some goods.
But I think we're shifting the discussion a bit to a "Long Emergency" scenario.
As a bit of localist, I'm more troubled by local governments who bend over backward to bring large retailers into an area. I think these more localized benefits (tax breaks, development dollars, etc) are far more beneficial to, say, Wal-Mart, than the highway system.
(1) And many that can have transformed that land into housing and strip malls.
