One of the major topics of discussion at the moment is the subject of control in marine traffic systems. For many years the principle of the freedom of the sea and in particular the freedom of navigation was recognized universally, but recent advances in many directions have led people to question it closely. The increasing size of ships, and especially those used to transport cargoes such as oil, chemicals and liquid natural gas, is one factor, since various incidents have led to a growing awareness of the human and ecological consequences of even a minor incident at sea. Another factor has been the expansion of oildrilling operations and explorations for other resources, usually in areas such as the North Sea where the available navigable sea room was already restricted, especially for the modern deeper-draught ships. Purely economic factors have also been contributory; modern ships spend less and less time in port with more time at sea and the commercial world demands that the sea voyages should be completed as efficiently as possible, usually with respect to time. For these and other local reasons there is a growing demand for some form of marine traffic control system. Most of the world's major ports have traffic control to some extent and attention is now being paid to restricted seaways such as the Dover Strait.