The highway trust fund was created in 1956 and is fed by the 18.4 cents-per-gallon tax on gasoline and a 24.3 cents-per-gallon tax on diesel.
Such taxes were assessed for 40 years before the trust fund but went into the general fund. Since 1956, it goes into the highway trust fund and if not spent on transportation projects stays in the fund, unavailable for other purposes.
At last a word of sanity on this subject. Alaska is always being accused of feeding at the trough, by people in states getting a million times more pork.
The story starts half way down the page. Its worth your time to read it.
The key point, This is Highway Trust Fund money, if not spent there, it does not buy anything else. It stays in the trust fund.
OK put up a toll bridge.
No, if not spent, it goes to pay for other highway projects benefiting more than 50 people.