(1) The system of prospective state highways heretofore selected by the county boards and approved by the highway commission are hereby validated but without prejudice to the exercise of the power to change such systems. Such systems are hereby designated as the county aid highway system.
(2) The department, on the petition of at least 100 freeholders, may, after investigation, make such alterations in the system of county aid highways as it deems necessary to serve the public interest.
(3) The county board may alter such systems as provided in s. 83.025 (1).