Veh. Code, § 22407

Whenever the Department of Transportation or local authority determines upon the basis of engineering studies and a traffic survey that the speed of 55 miles per hour is more than is reasonable or safe for vehicles mentioned in subdivision (a) of Section 22406, which have a manufacturer’s gross vehicle weight rating of 10,000 pounds or more, in descending a grade upon any portion of a highway, the department or local authority, with respect to highways under their respective jurisdiction, may determine and declare a speed limit of 50, 45, 40, 35, 30, 25, or 20 miles per hour, whichever is found most appropriate to facilitate the orderly movement of traffic and is reasonable and safe, which declared speed limit shall be effective for such vehicles when appropriate signs giving notice thereof are erected upon the highway.