Um. Nobody's Driving To Work
I think the biggest, er, driver of Americans driving less is the impact of taking a few million-odd people out of the workforce. I'd like to believe that increased consciousness of greenhouse gases or love of mass transit is a factor, but let's get real.
However, now would be the perfect time to increase gas taxes to more properly price the externalities of gasoline use, when it would affect less consumers and prices are relatively low. Plus, gas taxes fund transportation (mostly highways, but that could change), which means jobs. Not likely it will happen, however.