in many suburbs there are just no sidewalks, and busy multi lane roads.. So even if its under a mile away, you can't walk there.
If you are in and older town, with the old downtown, its not so bad. So most people in east coast cities, and older towns have access to walk into town. There are exceptions accross the contry.. However Any Town USA has its stores in a strip mall with a wallmart on a busy strode and no foot access.