I think this is a good point that gets overlooked alot. I've never been to Europe, yet, so somebody from across the pond please correct me if I'm wrong, but generally speaking Europe is much more interconnected than the US. There are vast expanses of mostly rural areas in the US/Canada where having a car is, for all intensive purposes mandatory.
The public transportation systems in the not-major US cities also, for the most part suck. NYC, Boston, Chicago, etc. all have fairly decent public systems of transport. However, in Rochester NY where I go to school, the public system is a joke. Buses run at extremely long intervals, and there are very few lines to begin with. And there is no subway or other system running with that.
A large problem in the US is suburban sprawl in relation to this. There are a LOT of people who drive 20-30 miles each direction to work, because they choose to live in an outlying area. I don't see this being feasible for much longer. In combination with the current housing crisis, I think that we're already beginning to see the failure of this system........ foreclosures on homes that are the "American Dream" type of residence are skyrocketing; my question is: where are these people going?