>Fair, but which countries? It may explain some of the huge disconnect I’m seeing (demographics).
I think a lot of places have them. Denmark, Belgium, Germany, Netherlands, the UK ... basically any European country. I mean it's not like every (or even most) one-way street is like that in those places, but they're common enough.
There are also "advisory cycle lanes", where there is physically one car lane, but cars can go in both direction. They are expected to (slowly and carefully!) go into the cycle lanes either side in order to pass each other. Speed limit is very low on these ofc.
>Would you agree, though, that in roads that are designed for cars (most of the US), that’s an obviously bad idea and recipe for getting maimed or killed?
Actually no, "roads designed for cars" tend to be pretty wide. Take an existing two-lane road, cut it down to a single lane down the middle and have half-width cycle lanes either side. It's easier to do that on a wide road than a narrow European city road. (American lanes are too wide anyway.)
The problem is not "roads designed for cars", it is politics designed for cars. The roads are not immutable, they can be modified, if there is political will. It can be done: Netherlands was rebuilt after WW2 to be car-dependent in the American pattern, and then that was undone in the 1970s onwards after the oil crisis and record pedestrian fatalities:
https://one-handed-economist.com/wp-content/uploads/2021/11/...
There is a myth that European cities have always been pedestrian and bike friendly because they inherit their street design from a time before cars -- not always so! Things have gone back and forth.