All ships at sea have to comply with a set of international rules about how to conduct themselves - no ,matter what nationality, size or means of propulsion - the rules are the same the whole world over - the ominous sounding Collision Regulations or "Rules of the Road" - aka the Highway Code of the sea.
As a rule, ships are free to roam anywhere they please by whichever course they care to choose. However, there are areas where shipping traffic has to comply when traveling in certain directions, normally off heavily congested transit points like those of the Scillies and Land's End.
In the example above, the small coaster Arklow Wind is on passage from Dunkirk In France to Aughinish in Ireland. In order to safely navigate past Land's End she has to comply with following the North bound sector of the traffic separation scheme. All those ships making their way North from the English Channel or coming up from the coast of France have to follow a narrow shipping lane closest to Land;s End, South bound traffic must take the western shipping lane. The AIS also shows how the boats must join and leave the separation lanes at their extremity.
Any vessel needing to cross these invisible lanes must do so, or as near as they can to right angles. Fishing vessels are also expected to follow the same procedure.
Here are the rules in full:
Traffic separation schemes
(a) This Rule applies to traffic separation schemes adopted by the Organization and does not relieve any vessel of her obligation under any other Rule.
(b) A vessel using a traffic separation scheme shall:
(i) proceed in the appropriate traffic lane in the general direction of traffic flow for that lane;
(ii) so far as practicable keep clear of a traffic separation line or separation zone;
(iii) normally join or leave a traffic lane at the termination of the lane, but when joining or leaving from either side shall do so at as small an angle to the general direction of traffic flow as practicable.
(c) A vessel shall, so far as practicable, avoid crossing traffic lanes but if obliged to do so shall cross on a heading as nearly as practicable at right angles to the general direction of traffic flow.
(i) A vessel shall not use an inshore traffic zone when she can safely use the appropriate traffic lane within the adjacent traffic separation scheme. However, vessels of less than 20 metres in length, sailing vessels and vessels engaged in fishing may use the inshore traffic zone.(e) A vessel other than a crossing vessel or a vessel joining or leaving a lane shall not normally enter a separation zone or cross a separation line except:
(ii) Notwithstanding sub-paragraph (d) (i), a vessel may use an inshore traffic zone when en route to or from a port, offshore installation or structure, pilot station or any other place situated within the inshore traffic zone, or to avoid immediate danger.
(i) in cases of emergency to avoid immediate danger;
(ii) to engage in fishing within a separation zone.
