California Code, Streets and Highways Code - SHC § 30861

The license tax and rate of toll as originally fixed by the board of supervisors shall not be increased or diminished at any time during the term of 20 years, unless it is shown to the satisfaction of the board of supervisors that the receipts from tolls in any one year is disproportionate to the cost of construction or erection, or the fair cash value thereof, together with the cost of all necessary repairs and maintenance of the bridge or ferry. The license tax fixed by the board shall not exceed 10 percent of the tolls annually collected.