It's a bit more complicated than that. The routes I'm talking about are the North Atlantic Tracks, which are used for most traffic between North America and Eur

It's a bit more complicated than that. The routes I'm talking about are the North Atlantic Tracks, which are used for most traffic between North America and Europe. There are multiple tracks and SLOP is used within each track. All of the tracks run in the same direction at the same time, switching directions twice a day. They go eastbound at night, westbound during the day. SLOP is a mitigation to prevent aircraft in sequence on the same track from colliding. There are, of course, many other systems and procedures in place to prevent such collisions, but it's belt-and-suspenders up there.