The director is hereby vested with complete authority to designate, locate, create, and determine what roads, highways, and streets constitute the state highway system, subject however, to such conditions, requirements, and mileage limits as provided for by law. The total mileage of the state highway system may be increased by not more than fifty miles [80.47    kilometers] in any one calendar year. In designating, locating, creating, and determining the several routes of the state highway system, the director shall take into account such factors as the actual or potential traffic volumes, the type of service class, the construction of bypasses and alternate routes, the conservation and development of the state’s natural resources, the general economy of the state and communities, and the desirability of fitting such system into the general scheme of the nationwide network of highways.