California Code, Vehicle Code - VEH § 21759

The driver of any vehicle approaching any horse drawn vehicle, any ridden animal, or any livestock shall exercise proper control of his vehicle and shall reduce speed or stop as may appear necessary or as may be signalled or otherwise requested by any person driving, riding or in charge of the animal or livestock in order to avoid frightening and to safeguard the animal or livestock and to insure the safety of any person driving or riding the animal or in charge of the livestock.