605 ILCS 5/5-401
Subject to the general supervisory powers of the Department under this Code, all highways in the county highway system shall be under the direct control and supervision of the county board of the county in which such county highways are located, and the county board shall repair, maintain and construct such county highways by contract or with its own forces.
However, gravel and macadam highways constructed or partially constructed prior to July 1, 1929 as State aid roads under the provisions of "An Act to revise the law in relation to roads and bridges", approved June 27, 1913, as amended, and required to be maintained equally by the county and the Department under the provisions of Section 32 of that Act shall continue to be so maintained.
605 ILCS 5/5-401