I am grading papers from students in my Advanced Urban course, and a number are reviewing literature on density and use of public transport. The literature suggests that doubling density is associated with something like an eight percent increase in public transit use, but of course, it is difficult to tease out cause and effect: I suspect those who like living densely are also more likely to want to use public transportation.
I can't help but think about a trip I made to a UN conference on urban issues. The conference took place in Barcelona, which has among the easiest to use transit systems in the world--more than half the people there live within walking distance of a metro stop. As it happens, I went to dinner with some officials from the Bush Administration, and when I suggested we use the metro instead of cabs, my companions were, well, stunned at the very idea. I pursuaded them to go, and learned that a bunch of people who lived in a city which has an excellent metro, Washington, never used public transportation.
I could be wrong, but my sense was that taking the metro in Barcelona was a foreign adventure for them in all kind of ways, and one that they did not particularly wish to repeat at home.