Doesn't matter what your speedometer says. Doesn't matter that you're exiting left in twelve miles or that the right lane has trucks in it. Somebody just went around you on the side nobody looks, and that's the road telling you something. Here's the rule, lane by lane, a simulation to prove it to yourself, and the excuses that don't hold up.
Three lanes, traffic moving left to right. Tap a lane to see its job.
You're here because someone in the middle is slower than you, and for no other reason. Get past them, then get out; all the way right if there's nobody else to pass. Someone filling your mirror? Then you're not the one passing anymore, you're the one being passed. Move over, whatever your speedometer says.
There's a slower car in the right lane and you're going around it. That's the whole reason to be here, and it takes about ten seconds. If you've been here longer than that and can't point at the car you're passing, you're camping, and everyone doing five over is now going around you on the right.
This is the lane. Not the slow lane, not the truck lane; the lane. If you aren't in the middle of getting around somebody, this is where you are. Merges near ramps are part of the deal; give them room and carry on. "Too many merges" is a reason to pass one car, not a reason to move in for the winter.
Two copies of the same highway loop with the same drivers: identical preferred speeds, identical starting spots. On the top road everyone follows the strict rule: drive right, pull out only to get past someone, come straight back the moment you're not passing anyone. On the bottom road it's a normal mix: a third keep right, a third pick a lane and sit in it, a third take whichever lane is moving and pass on either side. The bottom road is most highways. Push the sliders and watch the numbers.
Warming up…
Lane changes aren't bad by themselves. Passing means changing lanes at least twice, and the strict version comes back right every time, so the top road usually changes lanes more. The moves that cause trouble are passes on the right and cut-offs, which is why those get their own counters. Held up is how far below their preferred speed drivers are, on average.
When passing happens on the left and everyone else stays right, each lane ends up full of cars going about the same speed. The cars that interact with each other are the ones with the smallest speed difference, and speed difference between vehicles, more than speed itself, is what crash research keeps pointing at.
Drivers check the left mirror before drifting left. A car overtaking on the right is in exactly the spot they're least likely to look, and it's there because someone up ahead wouldn't move over. Predictable traffic is safe traffic: if every pass comes from the same direction, everyone can plan around it.
One slower car parked in the left lane means the passing lane no longer exists. Faster traffic stacks up behind it into a platoon, and when that platoon finally breaks it does so with a burst of lane changes and braking. Keep the lane open and the platoon never forms.
A lane you can pass in is a lane nobody is sitting in. Every car that "cruises" in the center makes the center useless for passing, which pushes every pass into the left lane, which is now the only lane doing the work of two. Come back right every time and both passing lanes stay open for the ten seconds each pass needs.
On a busy road a single brake tap travels backward from car to car and grows as it goes; researchers have produced stop-and-go jams on an empty circular track with nothing but cars following each other. Every surprise move is a tap. Removing the surprises removes waves before they start.
Yes. And since this page is fairly loud about it, here's the fine print, kept honest.