Gov. Code, § 25305

When in its judgment the public interest requires, the board may purchase automobiles for the use of county officers and county employees whose duties require frequent trips on county business. By ordinance the board shall adopt rules and regulations governing the use of the automobiles and the imposition of proper penalties upon any person driving, operating, or using the automobiles contrary to the rules and regulations of the board. The board may purchase necessary automobiles and assign them to county boards and commissions or members thereof whose duties require frequent and adequate transportation in and about the county on county business.

County business shall include the operation of county-owned or county-leased vehicles as commute vehicles in a carpool or vanpool program authorized by the county for county employees, provided that a daily, weekly, or monthly fee is charged that is adequate to reimburse the county for all costs of providing, operating and maintaining vehicles for such purpose. The board of supervisors shall establish rules, regulations and procedures to be used in the operation of county-owned or county-leased vehicles in a county carpool or vanpool program for county employees.

The use of automobiles provided by the county pursuant to this section is in lieu of any other mileage allowed by law. The board may allow officers and employees using county automobiles their actual and necessary expenses when traveling on county business.