Whenever the director with respect to highways and the superintendent of the North Dakota state highway patrol shall jointly determine upon the basis of an engineering and traffic investigation that the speed of vehicular traffic on a state highway is greater or less than is reasonable or safe under the conditions found to exist at any intersection or other place or upon any part of such highway to include streets within the corporate limits of any city, when such streets have been designated as part of any state highway, said officials acting jointly may determine and declare a reasonable and safe speed limit thereat not in excess of the maximum prescribed by law, which shall be effective at all times or during hours of daylight or darkness or at such other times as may be determined when appropriate signs giving notice thereof are erected at such intersections or other place or part of the highway.