Whenever the state road commissioner shall determine upon the basis of an engineering and traffic investigation that any speed limit set forth in this article 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 a highway, said commissioner may determine and declare a reasonable and safe speed limit thereat 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 intersection or other place or part of the highway.