Via Ryan Avent, Reuters reports that mass transit is gaining in popularity: "Mass transit use increased by more than 2 percent in 2007 to the highest level in 50 years, with Americans taking more than 10 billion trips on public transport while the number of vehicle miles traveled was flat in the first 10 months of the year." Light rail (including street cars) led the way, followed by commuter rail and then subways.
You sometimes hear discussions proceed in which decisions about transportation are portrayed as almost entirely inelastic. Often this involves the supposition that the entire population of the United States lives either in Manhattan or else Wyoming. In reality, lots of people -- including most New Yorkers -- live someplace in-between. If the negative externalities of driving aren't prices, and the federal government divvies up its spending 40:1 in favor of roads over transit, and local governments mandate that "free" parking be built everywhere then, naturally, people in those in-between places will tend to drive. But as gas prices go up, behavior does change.
If we priced gasoline consumption and road use properly and spent more money on improving existing transit infrastructure and building some new systems, then things would change even more. Even for people who live in pretty sprawled-out bits of suburbs and rely on cars to get them around their community can find a good commuter rail line to be an appealing means of getting to work.
Photo by me; available under a Creative Commons license