When vessels are required to adhere to a routing scheme involving long segments of one-way traffic, as in the Dover Strait, the overtaking of one ship by another can result in some degree of hazard and nuisance as well as loss of time. The frequency of such manoeuvres would appear to depend on a number of factors, such as the density of shipping in the lane, the domain size of individual vessels and the speed and lateral-distribution profiles of the traffic. From general considerations it seemed likely that the manner in which overtaking manoeuvres are made, with regard to the presence of other ships in the vicinity, might have some effect on their frequency and extent. It therefore seemed worthwhile to investigate theoretically any change or reduction which might result if overtaking ships were to take account of the presence and location of other vessels in the vicinity when the decision whether to pass to port or starboard of a ship being overtaken was about to be made.