Whenever the Department of Transportation determines upon the basis of an engineering and traffic survey that the safe and orderly movement of traffic upon any state highway which is a freeway will be facilitated by the establishment of variable speed limits, the department may erect, regulate, and control signs upon the state highway which is a freeway, or any portion thereof, which signs shall be so designed as to permit display of different speed limits at various times of the day or night. Such signs need not conform to the standards and specifications established by regulations of the Department of Transportation pursuant to Section 21400, but shall be of sufficient size and clarity to give adequate notice of the applicable speed limit. The speed limit upon the freeway at a particular time and place shall be that which is then and there displayed upon such sign.