The lanes
Well, here in Iowa (middle of US), highway 151 used to be 1 lane each way with those passing lanes up hills. These lanes are something like 30 miles apart. Luckily it's now 2 lanes each way the whole way (with 3 lanes for a few hills), because (having driven in a lot of the US), Iowans are probably the rudest drivers in the country when it comes to this kind of thing.
What was supposed to happen was a right lane would appear for slower traffic, and the slow traffic should follow the HUUUGE "slower traffic use right lane" sign, drive in that right lane, and merge back in at the end.
What ACTUALLY would happen is, about 10% of the time there'd be no traffic at all, no problem. About 40% of the time, there'd be traffic but everyone behaved themselves -- slower traffic moved right, faster traffic (if there was any) went by on the left. (This is what I do, even if there's no traffic I move right if I'm not actually passing somebody.) Again, no problem. About 30% of the time, some asshole that HAD been going about 10MPH *under* the speed limit would stay in the left lane and speed up to about 10MPH *over* the speed limit, to try to prevent anyone passing them, then slow back down to 10MPH under once the slow lane is done. Needless to say I'd have none of that and just blast past them on the right side. About 10% of the time, you'd have a left-lane-driver *plus* a right-lane-pacer that'd drive along in the right lane at 1 car length + 1 inch behind the left lane car. Needless to say, since they are then an uncivilized road obstruction I had no qualms just squeezing that 1 inch in front of them and then taking off for a proper pass. The last 10% of the time, the right-lane-driver will go JUST fast enough so they use the ENTIRE passing area to just barely squeeze by the left lane car, making sure nobody else has room to pass. It's never a lack of horsepower, they'll just be cruising along in top gear and making no effort to pass in a timely manner *or* drop back so people can go around, oblivious to other cars (and even semis -- lorries to you) tailgating them, flashing lights, and honking at them.
Like I say, luckily this road's been widened to 2 lanes each way (with a few areas having a 3rd lane "slow lane"), so this simply isn't a problem on 151 any more.