Cal. GOV § 8903

When traveling to and from a session of the Legislature, or when traveling to and from a meeting of a committee of which he or she is a member, or when traveling pursuant to any other legislative function or responsibility as authorized or directed by the rules of the house of which he or she is a member or by the joint rules, when that travel is by a common carrier of passengers, a Member of the Legislature shall be entitled to reimbursement for the actual costs of travel by the common carrier. If the member travels by other means and common carrier service is available and feasible he or she shall be reimbursed in the amount of the fare of available common carrier service. If common carrier service is unavailable or not feasible, a member shall be reimbursed at a rate not to exceed the rate established by the Department of Human Resources for the reimbursement of officers and employees of the state pursuant to Section 19820. No mileage shall be allowed or paid for travel in a conveyance owned or provided by and at the expense of a public agency. As used in this section, “common carrier” means carrier by aircraft, railroad, bus, or vessel.