Keep left unless overtaking (or right if you drive on the other side of road like the other half of the world).
A simple rule every one should be able to follow. At least everyone with a driving license.
A rule I never fully appreciated or was even that all aware of when I lived in America. Americans pass on the left. And the right. But you expect it. Besides the speed limit on the motorways (aka freeways, highways, etc) is set so low it doesn't really matter.
Lane discipline comes into its own in Germany and this is where I fully learned the importance of it. Germany has many motorways which have no speed limits. This is why they invented the Porsche and many other high performance vehicles capable of doing a leisurely Sunday drive at 120 mph.
And there are few other things that will teach you the value of lane discipline than to glance in the rear view mirror, see nothing and then have a blur of a Porsche pass you doing 140 mph. You learn to stay out of way. Unless of course you too own one of those works of art capable of going at the speed of blur.
In Germany everyone practices lane discipline. The same way they never cross the road unless they have the little green man.
In Italy they have lane discipline sorted out in that they don't have any lanes. You drive where ever, when ever you want to drive and stay the heck out of the way. Full stop. Which is the why they invented the Ferrari.
In Britain lane discipline is more of an aspiration. The British abhor the slovenly approach to lane discipline in the USA. They silently admire the strict adherence to the rules of the road in Germany and they are downright green with envy of the Italians lack of due care. But the stiff upper lipped Brits can't quite imagine anything themselves outside of simply sitting in the middle lane.
The Brits will never ever pass you on the wrong side of the road. That would be a sign of weakness. They will however do the entire journey in the middle lane to avoid being passed or merged into. They suffer fairly strong status anxiety and can't quite bear the fact that someone is passing them so they thinking nothing of holding up all the traffic travelling at 10 miles below the speed limit in the middle lane. I have sat in 10 miles of traffic jams with the slow lane being completely empty. No one wants to be seen over there.
And if like my husband you have a terminal case of speed anxiety you must be the fastest person on the road passing everyone you come upon in the middle lane.
So be warned if driving in the UK, the slow lane is for losers. The middle lane are for slightly less losers. And the fast lane is for the brave. And the stupid.