Veh. Code, § 22362

It is prima facie a violation of the basic speed law for any person to operate a vehicle in excess of the posted speed limit upon any portion of a highway where officers or employees of the agency having jurisdiction of the same, or any contractor of the agency or his employees, are at work on the roadway or within the right-of-way so close thereto as to be endangered by passing traffic. This section applies only when appropriate signs, indicating the limits of the restricted zone, and the speed limit applicable therein, are placed by such agency within 400 feet of each end of such zone. The signs shall display the figures indicating the applicable limit, which shall not be less than 25 miles per hour, and shall indicate the purpose of the speed restriction. Nothing in this section shall be deemed to relieve any operator of a vehicle from complying with the basic speed law.