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 or safe under the conditions found to exist at such intersections and it is declared that a reasonable and safe speed limit thereat, which shall be effective at all times or during hours of daylight or darkness or at the times specified herein and within those intersections herein designated, shall be as herein stated, which speeds so declared shall be effective at the time specified herein when appropriate signs are erected at such intersections or upon approaches thereto giving notice of the speed limit so declared thereat.