605 ILCS 5/3-108

Any highway constructed as a federal aid highway under the provisions of the Federal Aid Road Act shall be a part of the State highway system unless, under the provisions of Section 3-104 or 3-104.1 or similar provisions of this Code, there is an agreement or provision made for its maintenance by the county or municipality, in which case it shall be a part of the county highway system or municipal street system.