[donall]

I think the theory is that the middle lane is only used for overtaking slower-moving cars. Perhaps the Swedes, like the Germans, practice good lane discipline and stick to the right until they need to overtake.

I think this 2+1 wouldn't work in the US because (in my experience) very few people use the lanes correctly and they just distribute themselves evenly across all lanes, regardless of speed. That strategy does, indeed, lead to bottlenecks when the number of lanes reduces.

[vacri]

Here in Australia, 2+1 roads aren't very long - they're usually just long enough for a few cars to overtake a slowpoke, then a few kilometers further on you'll get another chance. You can't really cruise double-abreast on these roads, as they're not long enough for that.

Similarly you can't really bottleneck because the only input is a single-lane road - they're not long enough to add anything more than trivial traffic in rural areas.

[statusgraph]

This is normal in the US for roads in mountains (going uphill) and it generally works great.