The legal limit often has no relation to it and no basis in science or common sense. Not to mention a failure to account for progress in vehicle development, tire technology, etc.
The socially agreed upon limit is the legal limit.
The speed limit is often arbitrary and that's part of the problem. We become immune to posted limits because the ones posted on highways and major roads are obviously inappropriate and not updated as technology and zoning changes. Then, many drivers ignore them when they do matter e.g. neighborhoods, school zones, curves.
You're almost never going to be affected by someone simply speeding - it's drivers who speed _recklessly_. I'm fortunate enough to have never gotten into an accident even though I've driven all around the world. The person that's going to cause an accident is the one speeding AND weaving between cars or tailgating.
Either way, it's irrelevant. The point of the thread is that speeding tickets are little more than a financial instrument for local, and sometimes state, governments.
Now I must decide whether I care more about my life or the law. It's not a great position to be placed in because you need to arrive at your destination 30 seconds faster.