It is determined upon the basis of an engineering and traffic investigation that the speed permitted by state law outside of business and residence districts as applicable upon the following streets is greaten than is reasonable or safe under the conditions found to exist upon such streets and is declared that the prima facie speed limit shall be thirty-five or forty-five miles per hour as herein set forth on those streets or parts of streets herein designated at the times herein specified when signs are erected giving notice thereof.