It's nothing more than political smoke and mirrors. At least one candidate for the Presidency has proposed it. However it will save you all of 18.4¢ per gallon of gas, not 40¢ as v b has claimed. The Federal gasoline tax is 18.4¢ per gallon, not 40¢. At current prices, that's less than a 5% price cut.
Also bear in mind that it is a PROPOSAL by a candidate for political office. He has no authority to make it law on his own, either now or if he should win. Both houses of Congress would have to pass the bill and the current President would have to sign it into law. Given the state of the Federal deficit, it won't happen.