625 ILCS 5/1-126.1

The Department of Transportation may designate streets or highways in the system of State highways as follows: (a) Class I highways include interstate highways, expressways, tollways, and other highways deemed appropriate by the Department. (b) Class II highways include State highways and designated local roads not built to interstate highway standards that have at least 11 feet lane widths. (c) (Blank). (d) Non-designated highways include State highways not designated as Class I or II and local highways which are part of any county, township, municipal, or district road system not designated as Class II. Local authorities also may designate Class II highways within their systems of highways.