Cal. Veh. Code § 34501.6

The governing board of a local educational agency that provides for the transportation of pupils shall adopt procedures that limit the operation of schoolbuses when atmospheric conditions reduce visibility on the roadway to 200 feet or less during regular home-to-school transportation service. Operational policies for school activity trips shall give schoolbus drivers discretionary authority to discontinue schoolbus operation if the driver determines that it is unsafe to continue operation because of reduced visibility.