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Milton Keynes, in the UK, seems to have nailed this. It’s effectively a grid of roundabouts. When the roads are empty, you can race along at 60mph (legally). As soon as it starts to build, the road naturally slows to 40, then 30mph. No cameras etc needed.
It also has the red ways. You can walk most places, without having to cross a major road. It uses underpasses for pedestrians and bikes etc.
That last part is where you lost us. Unless we can legally murder someone with our SUV and call it an accident, we Americans won’t have it because it’s for commies.