California Code, Vehicle Code - VEH § 27803

(a) A driver and any passenger shall wear a safety helmet meeting requirements established pursuant to Section 27802 when riding on a motorcycle, motor-driven cycle, or motorized bicycle.

(b) It is unlawful to operate a motorcycle, motor-driven cycle, or motorized bicycle if the driver or any passenger is not wearing a safety helmet as required by subdivision (a).

(c) It is unlawful to ride as a passenger on a motorcycle, motor-driven cycles, or motorized bicycle if the driver or any passenger is not wearing a safety helmet as required by subdivision (a).

(d) This section applies to persons who are riding on motorcycles, motor-driven cycles, or motorized bicycles operated on the highways.

(e) For the purposes of this section, “wear a safety helmet” or “wearing a safety helmet” means having a safety helmet meeting the requirements of Section 27802 on the person's head that is fastened with the helmet straps and that is of a size that fits the wearing person's head securely without excessive lateral or vertical movement.

(f) This section does not apply to a person operating, or riding as a passenger in, a fully enclosed three-wheeled motor vehicle that is not less than seven feet in length and not less than four feet in width, and has an unladen weight of 900 pounds or more, if the vehicle meets or exceeds all of the requirements of this code, the Federal Motor Vehicle Safety Standards, and the rules and regulations adopted by the United States Department of Transportation and the National Highway Traffic Safety Administration.

(g) In enacting this section, it is the intent of the Legislature to ensure that all persons are provided with an additional safety benefit while operating or riding a motorcycle, motor-driven cycle, or motorized bicycle.