Cal. Veh. Code § 31301

(a) No person shall transport any explosive substance, flammable liquid, liquefied petroleum gas or poisonous gas in a tank truck, trailer, or semitrailer through the Caldecott Tunnel located on State Highway, Route 24, near the Alameda-Contra Costa County boundary, connecting Oakland with Contra Costa County in the East Bay area at any time other than between the hours of 3 a.m. to 5 a.m.
(b) The Department of Transportation may, in compliance with the requirements of Article 1 (commencing with Section 22400) of Division 11, determine and declare a reduced speed limit, lower than the maximum speed of 55 miles per hour, found most appropriate for traffic safety between the hours of 3 a.m. and 5 a.m.
(c) Nothing in this section shall be construed as a limitation or restriction on the power of the Department of Transportation, conferred by any other provision of law, to adopt regulations with regard to the movement of vehicles, including, but not limited to, tank truck vehicles transporting any cargo specified under subdivision (a) through the Caldecott Tunnel.
If, pursuant to any such other law, the Department of Transportation adopts or amends regulations after the effective date of this section, which adopted or amended regulations govern the movement of vehicles subject to subdivision (a), then on the operative date of those regulations, this section shall no longer be operative.