I think some motor company thought about doing this 30-40 years ago. It was about as popular as hemorrhoids. They quickly dropped the idea.
The number one problem, I believe, is that frequently, out of safety, people have to exceed the speed limit, sometimes just for a few seconds, otherwise because they think they *have* to.
“Oh, gee, the wife has gone into labor, but the speed limit en route to the hospital is only 25 mph. So, sorry, honey. I guess the baby will have to wait! It’s only 10 miles!”
Exceeding the speed limit is one of the primary ways to avoid other people’s accidents.