Whenever local authorities, including boards of supervisors, within their respective jurisdictions, determine upon the basis of an engineering and traffic investigation that the speed permitted under this article on any street, or any county road or any portion thereof, or at any intersection is greater than is reasonable or safe under conditions found to exist upon such street, or any county road or any portion thereof, or at such intersection, such local authorities shall determine and declare, by ordinance, a reasonable and safe speed limit, which shall be effective when appropriate signs giving notice thereof are erected on such street, or any county road or any portion thereof, or at such intersection, or upon the approaches thereto. However, no speed limit shall be fixed by any such local authorities at less than fifteen (15) miles per hour.

The Commissioner of Corrections is authorized to establish by regulation reasonable and safe speed limits upon the roads of the correctional facilities under his jurisdiction which shall be effective when appropriate signs giving notice thereof are erected. Speed limits may be based upon road or traffic conditions or upon security considerations.

Provided, however, that whenever the State Highway Commission shall, pursuant to Section 63-3-503, lower the maximum speed limit in response to federal laws, regulations or guidelines for purposes of energy conservation, local authorities, including boards of supervisors, shall immediately lower maximum speed limits on local highways, not to exceed a maximum speed of fifty-five (55) miles per hour.