California Code, Vehicle Code - VEH § 27002

(a) No vehicle, except an authorized emergency vehicle, shall be equipped with, nor shall any person use upon a vehicle any siren except that an authorized emergency vehicle shall be equipped with a siren meeting requirements established by the department.

(b) An authorized emergency vehicle may also be equipped with a Hi-Lo audible warning sound meeting requirements established by the department.

(c) A Hi-Lo warning sound may only be used to notify the public of an immediate evacuation in case of an emergency and is not a siren.  For the purposes of Section 21055, a Hi-Lo shall not be used in lieu of a siren if the sounding of a siren is reasonably necessary pursuant to that section.