Over the past few years several papers have been written based on the results of surveys of marine traffic in different parts of the world. The general knowledge of the overall picture of the behaviour of ships at sea has increased considerably but there are still many aspects which need to be explored. One of the main fields of interest of the Marine Traffic Research Unit at the City of London Polytechnic is the use that mariners make of available sea room. For any particular area the navigational difficulties are going to vary considerably in situations where ships enter the area at different points, travelling indifferent directions, from situations in which the ships tend to keep to a number of defined routes through the area. If ships do tend to adopt a routing structure through an area, the next question of interest is how wide the various routes are, given no physical limitations for their width. The behaviour patterns may vary for different types of ship, and speed is another variable which may affect the situation. Answers to all these questions would be needed if one wanted to analyse the efficiency and safety aspects of marine traffic in an area.