Of all forms of transport, ships are unique when it comes to avoiding one another. Compared with both land vehicles and aircraft they have remarkable freedom of action, so that collisions are avoided less by external regulation than by mariners exercising their individual skill. A framework for their actions is provided by the collision regulations which have evolved over hundreds of years, a process which seems unlikely to reach an end in the near future. Since it is reasonable that any changes should be informed by events, information on what mariners actually do to avoid one another can only contribute to the safety of ships at sea.

This paper is concerned with obtaining information on collision avoidance from radar records. It describes the data extraction and the selection of encounters between vessels, and presents some sample statistical results. Finally, probable applications of the information are discussed.