I'm a few days late on the assignment desk, but AJ points us to an interesting pdf with a graph comparing the density of various cities around the world. Cities in the US come in almost uniformly last. Click on it for a full size version:
Now, there's a level of density -- say, that of Bombay -- that's probably not desirable. But the fact that New York, our densest megacity, is is still quite low on the list, gives you an idea of how spread out the median American city is, and how much work will be required to either change those settlement patterns to better approximate world norms or graft a usable transit infrastructure onto our existing cities. The problem for folks worried about global warming is that our energy intensive lifestyle had a very long time to evolve, but a more conservatory approach has to be implemented pretty quickly.
Now, there's a level of density -- say, that of Bombay -- that's probably not desirable. But the fact that New York, our densest megacity, is is still quite low on the list, gives you an idea of how spread out the median American city is, and how much work will be required to either change those settlement patterns to better approximate world norms or graft a usable transit infrastructure onto our existing cities. The problem for folks worried about global warming is that our energy intensive lifestyle had a very long time to evolve, but a more conservatory approach has to be implemented pretty quickly. 
