Why speed kills
The reason is very simple. As speed rises, force of impact rises as the square of the speed. This is just physics. So, if you go from an impact at 20 to an impact at 40, the force of the impact does not double. It is 4 times as great.
This happens with something else. The human body, for rather obvious evolutionary reasons, is not damaged by impacts as a straight line function of their force. As the force of impact rises, the damage done rises as the square of the force. You can see why we would have evolved like this. We evolved in an environment in which low speed bumps and falls were common, and high speed ones rare.
This is why it is simple undisputable truth that if car hits pedestrian at 20mph, 90% will survive, and at 40mph 90% will die. It is the combination of a non linear rise in impact force on a non linear rise in susceptibility to damage.
There is empirical evidence that when speed bumps are installed and speed limited to 20mph, pedestrian deaths fall by about 70% and cyclist deaths by about one third.
We need to run the country for the convenience and safety of those who live and walk around and cycle in neighborhoods, not for the amusement of people who just want to drive through them as fast as possible. So the more speed cameras the better. The lower the limits the better.
You see, we do not care how much fun you have driving. We don't care if you don't like being careful to monitor your speed. We don't even care how fast you get from A to B. We just want you going through where we live very slowly and carefully. And if you won't do that, we want you off the road.