225 ILCS 440/4.01

Directional and other official signs, including, but not limited to, signs pertaining to natural wonders, scenic and historical attractions, which are required or authorized by law, and which comply with regulations which shall be promulgated by the Department relative to their lighting, size, number, spacing and such other requirements as may be appropriate to implement this Act, but such regulations shall not be inconsistent with, nor more restrictive than, such national standards as may be promulgated from time to time by the Secretary of the Department of Transportation of the United States, under subsection (c) of Section 131 of Title 23 of the United States Code.