The park district, city, village, or incorporated town may make such limit applicable at all times or only during certain specified times. Not more than 6 such alterations shall be made per mile along a highway or street; and the difference in limit between adjacent altered speed zones shall not be more than 10 miles per hour.
A limit so determined and declared by a park district, city, village, or incorporated town becomes effective, and suspends the application of the limit prescribed in Section 11-601 of this Chapter, when appropriate signs giving notice of the limit are erected at the proper place or along the proper part or zone of the highway or street. 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 of this Section 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 5/11-604