Not surprisingly, given the increase in public transit ridership, the Federal Highway Administration estimates that the number of miles logged on America's roads was down 3.7 percent in May (compared with May 2007). The sharpest drops were in Michigan (-7.4), Arkansas (-5.6), Rhode Island (-5.4), and Delaware, Indiana, and Vermont (-5.3 each). North Dakota was the only state where people drove more (up 0.7 percent); the smallest decreases where in Louisiana (-0.6) and Hawaii (-1.6).
I'm guessing the local economy is determining these rankings. People in distressed Michigan are probably not driving to jobs they no longer have; I don't think public transit is surging there. In Massachusetts (where T ridership is up but the economy is not as bad as in many Rust Belt states), drivership dropped by an estimated 3.9 percent, pretty close to the national average. I'm not sure why the decreases were sharper in Rhode Island and Vermont, which don't have extensive mass transit.
Kevin Drum blogs on the long-term driving trends here.