The Feds have been using a per-gallon tax to raise money since 1932 when it started at just a penny. President Eisenhower was the first to specifically earmark the revenue for roads in 1956 and used the cash to help fund the interstate system. After a few other increases, the last hike came under President Clinton. Of course, when figuring in inflation, those 18.4 cents buy less today than in '93.
A poll last year found that over half of the people surveyed would have been willing to pay a higher gas tax. So what's the hold up? According to ProPublica's piece, the hatred of any new taxation within Congress is largely the culprit. Republicans loathe the concept of raising the amount in general, and Democrats feel the fee on gas is regressive by having a greater effect on people with lower incomes. The whole article is a fascinating examination of the issue and well worth the time it takes to read.