605 ILCS 5/3-104
The Department in cooperation with the county boards and Federal Highway Administration is authorized to select and designate a network of federal aid secondary highways consisting of not more than 25,000 miles to be known as the federal aid secondary network. The highways included in such network shall be selected and designated in accordance with the provisions of the Federal Aid Road Act, and shall consist of highways not included in the federal aid primary or federal aid urban networks.
Any highway included in the approved federal aid secondary network may be constructed jointly at the expense of the federal government and the State of Illinois, or jointly at the expense of the federal government and a county or counties, or jointly at the expense of the federal government, the State of Illinois, and a county or counties, in accordance with the provisions of the Federal Aid Road Act.
The local highway authorities having jurisdiction over a highway or street prior to its selection and designation as a part of the federal aid secondary network shall continue to be responsible for its maintenance until such time as it has been constructed as provided herein. After a highway has been so constructed, the Department is authorized to maintain it, or, with the approval of the Federal Highway Administration, to enter into a formal agreement with the appropriate officials of the county in which such highway is located, either prior to or after construction, for its maintenance at the expense of such county. If at any time the Department finds that the highway is not being properly maintained by such county, the Department shall call such fact to the attention of the county and if within 90 days after receipt of such notice the highway has not been put in a condition of maintenance satisfactory to the Department and the Federal Highway Administration, then the Department, with the consent of the Federal Highway Administration, shall withhold approval of any further federal aid secondary projects in such county until such highway has been so restored.
605 ILCS 5/3-104