(625 ILCS 5/11-604) (from Ch. 95 1/2, par. 11-604)
    Sec. 11-604. Alteration of limits by local authorities.
    (a) Subject to the limitations set forth in this Section, the county board of a county may establish absolute maximum speed limits on all county highways, township roads and district roads as defined in the Illinois Highway Code, except those under the jurisdiction of the Department or of the Illinois State Toll Highway Authority, as described in Sections 11-602 and 11-603 of this Chapter; and any park district, city, village, or incorporated town may establish absolute maximum speed limits on all streets which are within its corporate limits and which are not under the jurisdiction of the Department or of such Authority, and for which the county or a highway commissioner of such county does not have maintenance responsibility.
    (b) Whenever any such park district, city, village, or incorporated town determines, upon the basis of an engineering or traffic investigation concerning a highway or street on which it is authorized by this Section to establish speed limits, 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 place or along any part or zone of such highway or street, the local authority or park district shall determine and declare by ordinance a reasonable and safe absolute maximum speed limit at such place or along such part or zone, which:
        (1) Decreases the limit within an urban district, but
    
not to less than 20 miles per hour; or
        (2) Increases the limit within an urban district, but
    
not to more than 55 miles per hour; or
        (3) Decreases the limit outside of an urban district,
    
but not to less than 35 miles per hour, except as otherwise provided in subparagraph 4 of this paragraph; or
        (4) Decreases the limit within a residence district,
    
but not to less than 25 miles per hour, except as otherwise provided in subparagraph 1 of this paragraph.
    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.
    (c) A county engineer or superintendent of highways may submit to the Department for approval, a county policy for establishing altered speed zones on township and county highways based upon engineering and traffic investigations.
    (d) Whenever the county board of a county determines that a maximum speed limit is greater or less than is reasonable or safe with respect to the conditions found to exist at any place or along any part or zone of the highway or road, the county board shall determine and declare by ordinance a reasonable and safe absolute maximum speed limit at that place or along that part or zone. However, the maximum speed limit shall not exceed 55 miles per hour. Upon receipt of an engineering study for the part or zone of highway in question from the county engineer, and notwithstanding any other provision of law, the county board of a county may determine and declare by ordinance a reduction in the maximum speed limit at any place or along any part or zone of a county highway whenever the county board, in its sole discretion, determines that the reduction in the maximum speed limit is reasonable and safe. The county board may post signs designating the new speed limit. The limit 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 of the 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 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.
(Source: P.A. 95-574, eff. 6-1-08; 95-788, eff. 8-7-08.)