625 ILCS 5/11-602
Whenever the Department determines, upon the basis of an engineering and traffic investigation concerning any highway for which the Department has maintenance responsibility, 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 the conditions found to exist at any intersection or other place on such highway or along any part or zone thereof, the Department shall determine and declare a reasonable and safe absolute maximum speed limit applicable to such intersection or place, or along such part or zone. However, such limit shall conform with the maximum speed limit restrictions provided for in Section 11-601 of this Code . Where a highway under the Department's jurisdiction is contiguous to school property, the Department may, at the school district's request, set a reduced maximum speed limit for student safety purposes in the portion of the highway that faces the school property and in the portions of the highway that extend one-quarter mile in each direction from the opposite ends of the school property. A limit determined and declared as provided in this Section becomes effective, and suspends the applicability of the limit prescribed in Section 11-601 of this Chapter, when appropriate signs giving notice of the limit are erected at such intersection or other 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. However, nothing in this Section prohibits the use of such electronic speed-detecting devices within 500 feet of a sign within a special school speed zone indicating such zone, conforming to the requirements of Section 11-605 of this Act, nor shall evidence obtained thereby be inadmissible in any prosecution for speeding provided the use of such device shall apply only to the enforcement of the speed limit in such special school speed zone.
625 ILCS 5/11-602