605 ILCS 5/8-101

The Department, the county board of any county, or the corporate authorities of any municipality are hereby authorized, when the safety and convenience of highway traffic will be promoted and the public interest subserved thereby, to designate and establish any existing or proposed highway under each of their respective jurisdiction and control and for the maintenance of which any one of which aforesaid governmental authorities is or will be wholly responsible as a freeway, and to plan, locate, relocate, construct, reconstruct, maintain, alter, improve, vacate and regulate the use of such freeway in the same manner as they now are or hereafter may be authorized by law relating to highways under their respective jurisdiction and control. The Department, the county board of any county, or the corporate authorities of any municipality are further authorized to include in the foregoing freeway designation such related portions of intersecting highways, roads, streets and other public ways not under their jurisdiction and control and for the maintenance of which they are not wholly responsible, as require such designation to promote the safety and convenience of highway traffic.