27400. 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:
(... snip ...)
(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.