Maybe this is a hijack but looking at speed as the safety control is out-of-date thinking. Cars are extremely safe and a difference of 20mph re: kinetic energy just does not make that much of a difference in almost all cases.
Don’t get me wrong. Looking at speed and reaction time is important where every extra mph works out to (approx) 1.5 feet per second of reaction time when kids may be running around or blind driveways empty onto a street, but the safety issues I see on the freeway is NOT everyone going 75 in a 65. It is people that no matter how fast traffic is going, they have to go 20 mph faster than everyone else and weave in and out, often missing people by inches, to get ahead of you and you and everyone else. It is trucks going 45 mph side by side in the number 2, 3 and 4 lanes that cause everyone to jam on their brakes and change lanes. It is people that try to merge 20 mph below the speed limit, or cars that refuse to give you a couple more feet to change lanes or cars that hang out in blind spots or the newest craze, you have plenty of room to merge so you hit you signal and the car intentionally speeds up because fuck you for trying to get in front of me. And of course the infamous you are going too slow (60 on a 55 county road) that I have to cross a solid line to pass you and do at least 70. Feel free to add your story here.
I bring it up in this thread because my commute takes me on county roads at 55 mph. A county road access to the freeway that is 45 mph and freeways that range from 55 to 75 mph speed limit where traffic under the right circumstances will do 90+ and at no point do I feel my safety changes because of the speed but rather I feel unsafe because of other drivers. I would feel perfectly safe driving 100 mph on a freeway if everyone was +/- 10% and driving safely.