California Code, Vehicle Code - VEH § 21712

(a) A person driving a motor vehicle shall not knowingly permit a person to ride on a vehicle or upon a portion of a vehicle that is not designed or intended for the use of passengers.

(b) A person shall not ride on a vehicle or upon a portion of a vehicle that is not designed or intended for the use of passengers.

(c) A person driving a motor vehicle shall not knowingly permit a person to ride in the trunk of that motor vehicle.

(d) A person shall not ride in the trunk of a motor vehicle.

(e) A person violating subdivision (c) or (d) shall be punished as follows:

(f) Subdivisions (a) and (b) do not apply to an employee engaged in the necessary discharge of his or her duty or in the case of persons riding completely within or upon vehicle bodies in the space intended for a load on the vehicle.

(g) A person shall not drive a motor vehicle that is towing a trailer coach, camp trailer, or trailer carrying a vessel, containing a passenger, except when a trailer carrying or designed to carry a vessel is engaged in the launching or recovery of the vessel.

(h) A person shall not knowingly drive a motor vehicle that is towing a person riding upon a motorcycle, motorized bicycle, bicycle, coaster, roller skates, sled, skis, or toy vehicle.

(i) Subdivision (g) does not apply to a trailer coach that is towed with a fifth-wheel device if the trailer coach is equipped with safety glazing materials wherever glazing materials are used in windows or doors, with an audible or visual signaling device that a passenger inside the trailer coach can use to gain the attention of the motor vehicle driver, and with at least one unobstructed exit capable of being opened from both the interior and exterior of the trailer coach.