§ 56. State to maintain roads improved by state appropriations under special laws. When any highway has been constructed or improved under a special law, with moneys taken from the state treasury and under plans prepared by a state department, the commissioner of transportation may at any time inspect such highway and if he determine it to be of sufficient importance and properly constructed, he may make an order directing that such highway become a part of the system of state highways in such county, and thereafter such highway shall be maintained as a state highway. Such order shall be served upon the chairman of the board of supervisors, and a certified copy thereof shall be filed in the office of the county clerk and one in the office of the state comptroller.