(a) Whenever the governing authority of an incorporated municipality or county, in its respective jurisdiction, determines on the basis of an engineering and traffic investigation that the maximum vehicle speed permitted under this chapter is greater than is reasonable and safe under the conditions found to exist upon a highway or part of a highway under its jurisdiction, such authority may determine and declare a reasonable and safe maximum vehicle speed limit thereon which:

(1) Decreases the limit at intersections;

(2) Decreases the limit outside an urban or residential district, but not to less than 30 miles per hour;

(3) Decreases the limit within an urban or residential district, but not to less than 25 miles per hour; or

(4) Decreases any speed limit where a special hazard or condition exists that requires lower speed for compliance with Code Section 40-6-180.

(b) Such an authority in its respective jurisdiction shall determine by an engineering and traffic investigation the proper maximum speed for all arterial streets and shall declare a reasonable and safe maximum speed limit thereon which may be greater or less than the maximum speed permitted under this chapter for an urban district, but in no case shall the maximum be established at higher than 55 miles per hour.

(c) Any altered limit established as authorized in this Code section shall be effective at all times or during hours of darkness or at other times as may be determined when appropriate signs giving notice thereof are erected upon such street or highway.

(d) Not more than six alterations as authorized in this Code section shall be made per mile along a street or highway, except in the case of reduced limits at intersections. The difference between adjacent limits shall not be more than ten miles per hour, except for reductions for school speed zones, which may be not more than 20 miles per hour when a warning sign is placed 700 feet in advance of the point at which the speed reduction is required.