The state highway commissioner, subject to the approval of the advisory board to the state highway commissioner and state administrative board, is hereby authorized and directed to lay out and establish not to exceed 500 miles of additional trunk line highways on such routes as are best adapted for serving the demands of public travel in various sections of the state. In addition, all roads that have been improved as federal aid projects, and all roads that have been, or that may hereafter be, approved for federal aid, shall be trunk line highways and subject to all provisions of law relating thereto.