Pay at the Pump

The Infrastructurist had a recent post on an IBM study that analyzed how high gas prices have to go before people start to use alternative means of transportation.

The >$4.00 prices we experienced last year seemed to be having some effect, but according to the study, the tipping point is $5.00 per gallon, which seems a bit remote at the moment.

What would it take to pass a "floating" gas tax that keeps the price at the pump at $4.50 or $5.00, with the revenue used for a combination of transit infrastructure and tax credits for those who absolutely must use fuel? I'm sure there's a lot more to making it work than that, but it seems like a logical idea to wean ourselves on purpose, rather than waiting until it's forced upon us.