Evidence obtained by county or municipal law enforcement officers in using speed detection devices within 300 feet of a reduction of a speed limit inside an incorporated municipality or within 600 feet of a reduction of a speed limit outside an incorporated municipality or consolidated city-county government shall be inadmissible in the prosecution of a violation of any municipal ordinance, county ordinance, or state law regulating speed; nor shall such evidence be admissible in the prosecution of a violation as aforesaid when such violation has occurred within 30 days following a reduction of the speed limit in the area where the violation took place. No speed detection device shall be employed by county, municipal, or campus law enforcement officers on any portion of any highway which has a grade in excess of 7 percent.