Cal. VEH § 21655

(a) Whenever the Department of Transportation or local authorities with respect to highways under their respective jurisdictions determines upon the basis of an engineering and traffic investigation that the designation of a specific lane or lanes for the travel of vehicles required to travel at reduced speeds would facilitate the safe and orderly movement of traffic, the department or local authority may designate a specific lane or lanes for the travel of vehicles which are subject to the provisions of Section 22406 and shall erect signs at reasonable intervals giving notice thereof.

(b) Any trailer bus, except as provided in Section 21655.5, and any vehicle subject to the provisions of Section 22406 shall be driven in the lane or lanes designated pursuant to subdivision (a) whenever signs have been erected giving notice of that designation. Except as otherwise provided in this subdivision, when a specific lane or lanes have not been so designated, any of those vehicles shall be driven in the right-hand lane for traffic or as close as practicable to the right edge or curb. If, however, a specific lane or lanes have not been designated on a divided highway having four or more clearly marked lanes for traffic in one direction, any of those vehicles may also be driven in the lane to the immediate left of that right-hand lane, unless otherwise prohibited under this code. When overtaking and passing another vehicle proceeding in the same direction, the driver shall use either the designated lane, the lane to the immediate left of the right-hand lane, or the right-hand lane for traffic as permitted under this code.

This subdivision does not apply to a driver who is preparing for a left- or right-hand turn or who is entering into or exiting from a highway or to a driver who must necessarily drive in a lane other than the right-hand lane to continue on his or her intended route.