At present all ports operate some form of traffic regulation within their limits. Harbour masters and port authorities may limit the passage of vessels either out of, or into, the port and special light or shape signals indicate to a vessel that such control is in operation. Dock and lock masters decide the priority of vessels through the entrance to closed systems. In channels outside the closed systems pilots and regular port users have developed standard patterns of behaviour for their mutual safety and convenience. Evidently this regulation is acceptable and as its development has been spontaneous it must have been considered necessary.