Whenever so requested by the governing body of a county, the Department shall install a system of highway name signs on state-maintained highways at such time and upon such terms and conditions as may be mutually agreed to between the county and the Commissioner of Highways. The Department shall install, using state forces or contract, the initial signing system, and the county shall be responsible for continuing maintenance of the signs. Supply of the signs by the Department, either by manufacture or purchase, and initial installation shall be paid for from funds available to the Department for highway maintenance. No highway funds shall be used by the county for the cost of maintaining the signing system.