California Code, Vehicle Code - VEH § 21707

No motor vehicle, except an authorized emergency vehicle or a vehicle of a duly authorized member of a fire or police department, shall be operated within the block wherein an emergency situation responded to by any fire department vehicle exists, except that in the event the nearest intersection to the emergency is more than 300 feet therefrom, this section shall prohibit operation of vehicles only within 300 feet of the emergency, unless directed to do so by a member of the fire department or police department, sheriff, deputy sheriff, or member of the California Highway Patrol. The emergency shall be deemed to have ceased to exist when the official of the fire department in charge at the scene of the emergency shall so indicate. Officials of the fire department or police department or the Department of the California Highway Patrol who are present shall make every effort to prevent the closing off entirely of congested highway traffic passing the scene of any such emergency.