Veh. Code, § 35404

Any county having a population in excess of 4,000,000 and having within its limits a natural island with an area in excess of 20,000 acres may, by ordinance, prohibit the use of any highway or lane, hereafter established in unincorporated area thereon, (1) by any vehicle exceeding an overall length of 170 inches and an overall width of 65 inches, or (2) by any such vehicle and all vehicles driven by internal combustion engines. Notwithstanding the provisions of Section 906 of the Streets and Highways Code, no such ordinance shall be enacted unless the board of supervisors shall have theretofore adopted, by a four-fifths vote, a resolution determining that the public convenience and necessity require that such highway or lane have a width of 35 feet or less and a roadway width of 22 feet or less.

Any ordinance enacted pursuant to this section shall be subject to Sections 35718 to 35720, inclusive, of this code and shall not apply to authorized emergency vehicles.