California Vehicle Code
A person operating a motor vehicle or bicycle may not wear a headset covering, or earplugs in, both ears. This prohibition does not apply to any of the following:
(a) A person operating authorized emergency vehicles, as defined in Section 165.
(b) A person engaged in the operation of either special construction equipment or equipment for use in the maintenance of any highway.
(c) A person engaged in the operation of refuse collection equipment who is wearing a safety headset or safety earplugs.
(d) A person wearing personal hearing protectors in the form of earplugs or molds that are specifically designed to attenuate injurious noise levels. The plugs or molds shall be designed in a manner so as to not inhibit the wearer's ability to hear a siren or horn from an emergency vehicle or a horn from another motor vehicle.
(e) A person using a prosthetic device that aids the hard of hearing.