(a) Whenever local authorities within their respective jurisdictions determine upon the basis of an engineering and traffic investigation that the prima facie speed permitted under this subchapter at any intersection is greater than is reasonable or safe under the conditions found to exist at the intersection, then the local authority shall determine and declare a reasonable and safe prima facie speed limit, which shall be effective when appropriate signs giving notice are erected at such intersection or upon the approaches thereto if approved by the State Highway Commission.
(b) Local authorities in their respective jurisdictions may, in their discretion, authorize by ordinance higher prima facie speeds than those stated in § 27-51-201 upon through highways or upon highways or portions thereof where there are no intersections or between widely spaced intersections, if signs are erected giving notice of the authorized speed, but local authorities shall not have authority to modify or alter the basic rule set forth in § 27-51-201(a) or in any event to authorize by ordinance a speed in excess of forty-five miles per hour (45 m.p.h.).