The Charlotte Observer has an excellent piece on the editorial page from a couple of weeks ago about the gas tax and the state highway budget funds and how they are used. It gives a lesson of fairly recent legislative history that serves to shine a light on the ignorance of many critics of the gas tax and how the money is used by the legislature.
Revenues for roads/highways goes into one of two funds in the state budget. The Highway Fund is used to pay for general maintenance and repairs of state-owned roads. It also pays for some smaller construction projects and transportation programs. Revenues that support this fund come from the state motor fuel tax and vehicle registration fees. No money from this fund is transferred to the General Funds in the state budget.