605 ILCS 5/3-104.2
The Department, or the Department in cooperation with municipal corporate authorities or county authorities, is authorized to receive and expend, by agreement or otherwise, federal funds for the construction, reconstruction and improvement of public roads not on any Federal-aid system, being improved under the Federal Aid Road Act.
Any Federal-aid project as authorized in this Section may be undertaken jointly at the expense of the federal government and the State of Illinois, or jointly at the expense of the federal government, a municipality or municipalities, or a county or counties, or jointly at the expense of the federal government, the State of Illinois, a municipality or municipalities or a county or counties, or any of the above jointly with any other public or private entity or public utility.
The authority having jurisdiction over a public road not on any federal-aid system prior to its improvement as a federal-aid project shall continue to be responsible for maintenance, at its own expense.
For the purposes of this Section, "public road" means any road under the jurisdiction of and maintained by a public authority and open to public travel and which is not on a federal-aid system.
605 ILCS 5/3-104.2