And in case you're not sure what I'm talking about, take a gander at this:
"From 1932, when Congress first enacted an excise tax on gasoline, until 1956, the proceeds of the gas tax went into general revenues, although the amount raised each year was used as an informal benchmark for Federal highway spending. The Federal-Aid Highway Act of 1956 established the Highway Trust Fund and stipulated that 100% of the gas tax be deposited into the fund. From 1956 to 1982, the Highway Trust Fund was used solely to finance expenditures from the federal highway program.

Highway Trust Fund revenues were first allocated to mass transit in the Surface Transportation Act of 1982, when Congress raised the gas tax from four cents per gallon to nine cents per gallon and dedicated one cent, or 20 percent, of the increase to the newly-established Mass Transit Account. Each time there has been an increase in the amount of gas tax going into the Highway Trust Fund—1990, 1993 and 1997—20 percent of the increase has been allocated to the Transit Account and 80 percent to the Highway Account. Of the current gasoline tax of 18.3 cents per gallon, 2.86 cents per gallon is allocated to the Mass Transit Account. "

Highway Trust Fund:
"The Highway Trust Fund (HTF) was created by the Highway Revenue Act of 1956 (Pub. L. 84-627), primarily to ensure a dependable source of financing for the National System of Interstate and Defense Highways and also as the source of funding for the remainder of the Federal-aid Highway Program"