Cal. Sts. & High. Code § 25261

During the course of the improvement of any highway, the highway or project shall be under the control of the board of directors. The board of directors may adopt necessary regulations, not in conflict with law, concerning the use of the highway. During that period, the board may exclude from the highway any vehicular or other traffic which, in the opinion of the board of directors, may be detrimental to the public safety or will result in injury to the roadway during the progress of the improvement.