From the California Vehicle Code
CVC Section 27803 States:
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
(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
(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.
This section of the CVC along with Section 27802 is the basis of the CA Motorcycle Helmet Law.