605 ILCS 5/5-406
Where the county board of any county deems it necessary to connect any county highway with any county or State highway situated within an adjoining county, or to construct an interchange at the intersection of any county, State, or interstate highway situated within an adjoining county, the county board may by resolution of record request the county board of the adjoining county to improve such connection to the county line or construct such interchange, and the county board making said request is authorized to turn over to the adjoining county making such improvement or constructing the interchange, such part of the cost of the improvement or interchange as may be agreed upon between such counties, subject to the approval of the Department.
605 ILCS 5/5-406