Cal. Veh. Code § 21753

Except when passing on the right is permitted, the driver of an overtaken vehicle shall safely move to the right-hand side of the highway in favor of the overtaking vehicle after an audible signal or a momentary flash of headlights by the overtaking vehicle, and shall not increase the speed of his or her vehicle until completely passed by the overtaking vehicle. This section does not require the driver of an overtaken vehicle to drive on the shoulder of the highway in order to allow the overtaking vehicle to pass.