Vehicles shall not stop, stand or park except (a) when necessary to avoid injury or damage to persons or property, (b) while paying tolls, (c) in obedience to traffic signals or to an order, signal or direction of any member of the toll force, police department or (d) when unable to proceed because of the disability of the vehicle; provided, however, that vehicles may be parked in such places as may be designated by the manager or his duly authorized representative. Repairs and tire changes shall not be made except when authorized by a member of the toll force and under his direction. Stopping to pick up or discharge passengers is expressly forbidden except in designated parking areas.