Cal. Veh. Code § 23116

(a) No person driving a pickup truck or a flatbed motortruck on a highway shall transport any person in or on the back of the truck.
(b) No person shall ride in or on the back of a truck or flatbed motortruck being driven on a highway.
(c) Subdivisions (a) and (b) do not apply if the person in the back of the truck is secured with a restraint system. The restraint system shall meet or exceed the federal motor vehicle safety standards published in Sections 571.207, 571.209, and 571.210 of Title 49 of the Code of Federal Regulations.
(d) Subdivisions (a), (b), and (c) do not apply to any person transporting one or more persons in the back of a truck or flatbed motortruck owned by a farmer or rancher, if that vehicle is used exclusively within the boundaries of lands owned or managed by that farmer or rancher, including the incidental use of that vehicle on not more than one mile of highway between one part of the farm or ranch to another part of that farm or ranch.
(e) Subdivisions (a), (b), and (c) do not apply if the person in the back of the truck or the flatbed is being transported in an emergency response situation by a public agency or pursuant to the direction or authority of a public agency.
As used in this subdivision, “emergency response situation” means instances in which necessary measures are needed in order to prevent injury or death to persons or to prevent, confine, or mitigate damage or destruction to property.
(f) Subdivisions (a) and (b) do not apply if the person in the back of the truck or flatbed motortruck is being transported in a parade that is supervised by a law enforcement agency and the speed of the truck while in the parade does not exceed eight miles per hour.