625 ILCS 5/11-603
Whenever the Illinois State Toll Highway Authority determines, upon the basis of an engineering and traffic investigation concerning a toll highway under its jurisdiction, that a maximum speed limit prescribed in Section 11-601 of this Chapter is greater or less than is reasonable or safe with respect to conditions found to exist at any place or along any part or zone of such highway, the Authority shall determine and declare by regulation a reasonable and safe absolute maximum speed limit at such place or along such part or zone, and the speed limit shall conform with the maximum speed limit restrictions provided for in Section 11-601 of this Code . A limit so determined and declared becomes effective, and suspends the application of the limit prescribed in Section 11-601 of this Chapter, when (a) the Department concurs in writing with the Authority's regulation, and (b) appropriate signs giving notice of the limit are erected at such place or along such part or zone of the highway. Electronic speed-detecting devices shall not be used within 500 feet beyond any such sign in the direction of travel; if so used in violation hereof, evidence obtained thereby shall be inadmissible in any prosecution for speeding.
625 ILCS 5/11-603