Cal. Pub. Util. Code § 5401

Charges for the transportation to be offered or afforded by a charter-party carrier of passengers shall be computed and assessed on a vehicle mileage or time of use basis, or on a combination thereof. These charges may vary in accordance with the passenger capacity of the vehicle, or the size of the group to be transported. However, no charter-party carrier of passengers shall, directly or through an agent or otherwise, nor shall any broker, contract, agree, or arrange to charge, or demand or receive compensation, for the transportation offered or afforded that shall be computed, charged, or assessed on an individual-fare basis, except schoolbus contractors who are compensated by parents of children attending public, private, or parochial schools and except operators of round-trip sightseeing tour services conducted under a certificate subject to Section 5371.1, or a permit issued pursuant to subdivision (c) of Section 5384.