California Code, Public Utilities Code - PUC § 5371

No charter-party carrier of passengers excepting transit districts, transit authorities or cities owning and operating local transit systems themselves or through wholly owned nonprofit corporations shall engage in transportation services made subject to this chapter without first having obtained from the commission a certificate that public convenience and necessity require the operation, except that certain specific transportation services as defined in Section 5384 may be conducted under authority of a permit issued by the commission.