S.C. Code Ann. § 56-5-1540 – Alteration of speed limits by local authorities; signs; approval by Department of Transportation

(a) Establishing speed zones. Whenever local authorities in their respective jurisdictions determine on the basis of an engineering and traffic investigation that the maximum speed permitted under this article is greater or less than is reasonable and safe under the conditions found to exist upon a highway or part of a highway, the local authority may determine and declare a reasonable and safe maximum limit thereon which:
1. (1) decreases the limit at intersections; or
2. (2) increases the limit within an urban district but not to more than seventy miles an hour; or
3. (3) decreases the limit outside an urban district, but not to less than thirty-five miles an hour.
(b) Setting maximum limits on arterial streets. Local authorities in their respective jurisdictions shall determine by an engineering and traffic investigation the proper maximum speed for all arterial streets and shall declare a reasonable and safe maximum limit thereon which may be greater or less than the maximum speed permitted under this article for an urban district.
(c) Signs. Any altered limit established as hereinabove authorized is 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 the street or highway.
(d) Approval of altered limits by Department of Transportation. Any alteration of maximum limits on state highways or extensions thereof in a municipality by local authorities is not effective until the alteration has been approved by the Department of Transportation.
(e) Limitations on alterations. Not more than six such alterations as authorized above may be made for each mile along a street or highway, except in the case of reduced limits at intersections, and the difference between adjacent limits must not be more than ten miles an hour.