The ever lovable James Kunstler (last entry I looked at: Puke Town) believes that as our oil runs out we will be forced back into small towns and communities much like the United States in the 19th Century. Or possibly more accurately 16th century England.
In my opinion, a more likely eventuality for the greatly spread out United States is that much like the rest of the third world, there will be a pouring of population into the major economically active cities. It is more likely that the U.S. has some Mega-Cities in its future.
A rise in oil prices will make many currently (at least marginally) economic activities in the small towns too expensive to maintain. In Argentina, with the recent economic collapse, the railroads dropped service too many outlying towns(Argentinian Ghost Towns translated by Google)(translated by Google); Argentinian Ghost Towns (Spanish) ; Hat Tip FerFal )
The strong migration in the villages which are designed to disappear in different regions of the country is due to multiple causes, including the following: Closed railway stations, lack of investment in improving roads, lack of comprehensive studies of regions enable land use planning, isolation caused by the route of paved roads away from the old dirt roads, loss of the principal economic activity that gave life to the people and lack of service infrastructure following the downsizing of the people, lack of transport public to allow the relocation of the existing population, lack of public investment in formal and informal education, lack of jobs and lack of access to information and opportunities in general.As the article notes, a few people do hang on to these ghost towns, but they live an early version of the post apocalyptic life. Most prefer at least a possibility of services, if not opportunity in the big cities.
At the moment, there is a little bit of a spreading out trend within the United States. Some of my neighbors (around 4% of households) telecommute to work. That may still be possible, but only from areas that have other reasons to sustain a population density.
What has already happened, or is currently happening may not be what will happen in the future: but at least we know that it is possible for it to happen.