It is determined upon the basis of an engineering and traffic investigation that the speed permitted by state law at the following street intersections is greater than is reasonable on safe under the conditions found to exist at such intersections and is declared that the prima facie speed limit within one hundred feet upon every designated approach to and within those intersections herein designated shall be as herein stated, which speeds so declared shall be effective at the times specified herein when signs are erected upon every approach to every such intersection giving notice of the prima facie speed limit so declared thereat.