It is determined upon the basis of an engineering and traffic investigation that the speed permitted by state law upon the following streets is less than is necessary for safe operation of vehicles thereon by reason of the designation and signposting of such streets as through and/or by reason of the widely spaced intersections, and it is declared that the prima facie speed limit shall be as hereinafter set forth on those streets or parts of streets herein designated at the times specified when signs are erected giving notice thereof.