At some point, this becomes like asking how expensive cigarettes need to get before people will stop smoking. Whenever there's a big jump in a short time, the casual users will jump ship for a while, but the confirmed addicts will give up their apartments and live in their cars before they stop driving. The other thing I've noticed is that gas prices are like the frog in slowly warmed water. If gas prices go up slowly, even the casual users adjust their budgets accordingly.

That said, I do think commuting is up.