Lots of American cities are very spread out so you don't have to be in a rural area in order to have to travel more than five miles to satisfy your every day needs.
This is the major problem with transit in a lot of places. Development spread outward to cheaply priced land, creating large distances between residential areas and commercial areas. Major cities that didn't have the land or prices to allow outward development instead developed upward (and download, with buildings and transit systems below ground). The end result is that a lot of places are simply not sustainable without tremendous investment in their transit systems and city planning. As gas more and more expensive, a lot of people will end up having to abandon these areas because it will simply be too expensive to live there.