80% of deaths occur at speeds higher than 40mph. In a sane world that cared about human life speed limits would be 40mph and enforced. If I were king of the world it would be so.
Then there would be a study showing that most deaths occur over 25 mph, and you would lower the limit to 25.
Then a new study would come out showing most deaths occur at over 10 mph, and then we might as well walk.
Yes, there is some correlation between speed and severity of injury. Are you aware that there is also a correlation between speed and likelihood of accident?
Really, the faster you go, the less likely you are to be involved in an MVI. *IF* you are incompetent or unlucky and do crash, the results will be more devastating, but you are less likely to be involved in an incident above 70MPH than at 30. Really.