system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are Roundabouts a common traffic control feature in Europe but not in America?
gpt
If you look at a map of a typical American city and compare it to a European city, you will see Americans favor grid layouts while Europeans are more...organic. This is an artifact of history - most American cities were consciously settled as such and planned, whereas European cities often developed through history. 

This starts to become important which you consider that a grid layout will have far, far more intersections than the more spaghetti-like layout in Europe. Further, the grid offers traffic more options, whereas in most small and medium European towns there is really only one way in, out or through, so traffic gets concentrated on certain streets more. 

These two factors make the trade offs associated with a roundabout (uses more real estate, improves traffic flow) much more attractive in European city plan than an American one. It's also not a coincidence that the roundabouts you do see in the US are in its oldest areas. 

Bottom line is similar to high-speed rail, the US isn't just stubborn or stupid, the proposition simply isn't as attractive due to how it is laid out. 

Source: lived on both American coasts and now in Europe.