Except for highway construction zones, no street, road, or highway in the state highway system or any other township, county, or state road or highway may be posted in a manner which reduces the maximum speed limit on the street, road, or highway by more than twenty miles [32.19 kilometers] per hour between any two signs so posted in a speed zone. The maximum speed limit reduction between any two signs posted in a highway construction zone may not exceed thirty miles [48.28 kilometers] per hour.