The risk of a close quarters situation arising when a vessel crosses a busy traffic lane, such as one of the separation lanes in operation in the vicinity of the Dover Strait, is usually assessed in terms of the number of encounters or domain infringements likely to occur if no avoidance action were taken. Such calculations, which are based on the theoretical encounter rate, do not normally take into account any enhancement of the risk which may be associated with the simultaneous interaction of a crossing vessel with more than one other ship.