225 ILCS 440/3.11
"Unzoned commercial or industrial area" means any area adjacent to the right-of-way of a primary highway not zoned by any county or municipality and which lies within 600 feet of any commercial or industrial activity. All measurements shall be from the outer edges of the regularly used buildings, parking lots, storage or processing areas of the activities, not from the property lines of the activities, and shall be along or parallel to the edge or pavement of the highway. On primary highways other than expressways where there is an unzoned commercial or industrial area on one side of the road in accordance with the preceding, the unzoned commercial or industrial area shall also include those lands directly opposite on the other side of the highway to the extent of the same dimensions except where such lands are publicly owned or controlled for scenic or recreational purposes.
225 ILCS 440/3.11