This paper outlines the concept of a domain and an evasion area, called an arena, around a ship which are then applied to produce a computer model of ship behaviour. The arena determines when a ship takes avoiding action, as does the land arena which reacts with a discrete series of coastal points to prevent the ship running aground.

The increase in the number and size of ships has resulted in the introduction of traffic routing schemes and the need to understand ship behaviour more thoroughly. The concept of ‘the effective area around a ship which a navigator would like to keep clear with respect to other ships and stationary objects’ has been used by various authors including Goodwin, Fujii and Lewison with varying names such as domain, collision diameters and encounter area. There has been no fixed shape for these areas. Some are circular, others elliptical, while Goodwin's has three segments each with its own portion of a circle. By developing the theory of the domain, it was hoped to be able to produce a model of traffic behaviour which could be used to simulate traffic flows, or specific incidents, in order to study them more fully.