A recent study by digital computer simulation modelling of traffic interactions in the Dover Strait by Batkin made use of a so-called ‘decision-distance’ criterion for locating the point at which a crossing manoeuvre was to begin. This decision-distance was measured along the course of the vessel involved. The manoeuvring vessel was regarded as making an abrupt change of course, by an angle β to starboard on reaching the decision-distance, it having been ascertained beforehand that, in the absence of a manoeuvre, the vessel having the right of way would infringe the other's domain. Although it seems unlikely that such a simple procedure is used by mariners in practice, it seemed useful to examine the implications of the method in terms of the angles of manoeuvre involved, and to make a comparison with an alternative procedure involving the range between ships.