The first point to note about the North Atlantic air traffic system is the diurnal pattern of flow. This is strongly tidal, with westbound flow predominant during daylight hours and eastbound flow overnight. There is some traffic which flows against the main stream, particularly during the day, but the strong tendency for one-way traffic results from the difference in local times on opposite sides of the ocean, together with the transit times of flight at current speeds. This means that the general pattern will probably persist, unless and until aircraft speeds change radically.