At such times and under such conditions as may be authorized by the Safety Director private cars may be used by the Chief or by patrolmen in the line of duty. When in use in the line of duty, such vehicles shall be considered to be police vehicles and their drivers shall be considered immune from prosecution in those situations in which they would have been immune had they been driving a car owned by the City. When not used in the line of duty, drivers of private cars shall not operate or cause or allow to be operated any flashing or oscillating lights which might have been mounted on the cars in such position as to be seen by other motorists or pedestrians.