Cal. Veh. Code § 22107 : CALIFORNIA VEHICLE CODE — RULES OF THE ROAD — TURNING AND STOPPING AND TURNING SIGNALS — Turning from direct course or moving left or right upon roadway

No person shall turn a vehicle from a direct course or move right or left upon a roadway until such movement can be made with reasonable safety and then only after the giving of an appropriate signal in the manner provided in this chapter in the event any other vehicle may be affected by the movement.