> Why aren't cars speed limited to 80 mph? There is no road in the US where cars can go faster than that.

Unfortunately the cat's out of the bag on this one. Whoever limits first will be a massive safety hazard, not just to themselves but also to others. You must be able to match the speed of the other cars on the road, even if they're driving an illegal speed. Most deaths by speeding are more accurately described as deaths by speed differential, because they're a result of the speeding car encountering(and crashing into or losing control while avoiding) slower-moving obstacles. If all the cars on the road are going 70 in a 55, it's actually safest for everybody if you also match that speed, because then traffic is moving freely at a steady rate instead of lanes moving at different rates based on their slowest cars(and lots of slow-moving cars moving out of the slower lane into the faster lane).

To safely make your change, you'd need to shut down driving 100%, upgrade at least a critical mass of cars with this safety feature(you'd probably need to hit 90%+, to account for the statistical bias from people who don't normally go that fast suddenly driving their un-upgraded cars fast on the highway while they still can), and only then make it legal to drive again. That's...not going to happen. Not to mention the fact that speeders gonna speed, so you'd see people modding their cars to remove the limit immediately, and then we're right back to the status quo of there being a few cars out there that insist on driving 20+ faster than everybody else.