The commission is authorized to let the privilege of erecting, constructing and maintaining (during the period for which such privilege may be let) marking signs, guide boards and danger or warning signals with advertisements thereon, on and along the state highway system, at such points and places as may be designated by the commission, and all money received for such privilege shall be paid into the state treasury to the credit of the state road fund and may be used for maintenance purposes on the state highway system.