The department is hereby authorized to establish a continuing system for the designating of state highways and branches or portions thereof, heretofore established by the legislature of the state of Washington, to give designations to such state highways and branches, or portions thereof, in accord with that system, and to install signs in accord therewith on such state highways and branches, or portions thereof. The system may be changed from time to time and shall be extended to new state highways and branches, or portions thereof, as they are hereafter established by the legislature.