The problem is that at $4 a gallon, gasoline is still incredibly cheap. Most people can drive a fairly nice car for less than a $500 a month investment. $5 gas only jacks that up a little bit. That means that most people on even a lower middle class income can afford to drive to their heart's content. Add the fact that America is a very spread out place, and nothing but driving will make sense for a long time, even in big cities.
If and when gas hits $10 or so a gallon, lots of infrastructure improvements will happen. Which is the biggest reason I'm opposed to things like fracking. Just because we can squeeze more juice from the ground, doesn't mean it's good for us. That goes both for the environment, and research and development. I think we'd be a lot better off as a country now and in the future if we were forced to make next gen changes regarding energy and transportation. Tax subsidies don't do that as much as high energy prices do.