Americans are far less likely to use public transportation than residents of other countries, according to the National Geographic Society's 2009 Greendex report. Only five percent of Americans surveyed report daily use of public transportation, and only seven percent report taking public transportation at least once a week. 61 percent of Americans report that they never use public transportation.
Internationally, 25 percent of respondents in the 17-nation survey report using public transportation daily, and 41 percent report using it at least once a week. Even Canadians are more than twice as likely to report weekly or more transit usage than Americans; Germans are almost five times more likely to use transit at least weekly, and Russians are over ten times more likely.
You can calculate your own personal "greendex" here, and find out how you can improve your score. I haven't filled out the questionnaire yet, but think it involves things like doing more backpacking and less cooler-campering.