Ga. Code § 40-14-9
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, except that this 30 day limitation shall not apply to a speeding violation within a highway work zone, as defined in Code Section 40-6-188, or in an area with variable speed limits, as defined in Code Section 40-6-182. 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.
OCGA § 40-14-9