620 ILCS 5/49.1
No person may create or construct any airport hazard which obstructs a restricted landing area or residential airport that (1) serves 20 or more based aircraft, and (2) is located within the "metropolitan region" as that term is defined in the Regional Transportation Authority Act. For the purpose of this Section, "based aircraft" are aircraft that are regularly hangared or tied-down at the restricted landing area or residential airport, or that use it as their primary base of operation. As used in this Section 49.1, "restricted landing area" or "residential airport" shall have the meaning set forth in regulations of the Department in effect on the effective date of this amendatory Act of 1989, but shall not include amendments of the regulations adopted by the Department thereafter.
620 ILCS 5/49.1