People think a crash is about how fast you were going. It is about how fast you stopped.
Delta-V is the change in velocity during the impact, the speed your body actually gave up in the hundredths of a second the collision lasted. That is the number that predicts injury, not the number on the speed limit sign. And it does not climb in a straight line. Energy rises with the square of speed, so a modest jump in delta-V is a large jump in what the body has to absorb. The government's own injury-probability curves rise with delta-V, steeply.
Here is what that does inside a person. The car stops against the other object, the seatbelt stops the skeleton, and the organs keep moving on their tethers. The aorta, anchored at its arch and loose below, can shear. The brain rebounds off the inside of the skull. The liver and the spleen tear where they are attached. None of that needs a crushed cabin. It is why a wreck that looks survivable from the roadside, intact car, driver walking around, can turn fatal hours later from an injury no one saw.
The practical point: the story of a crash is not told by the dent or the speed limit. It is told by delta-V, and delta-V is read from the event data recorder. That download is the difference between a guess and a number, which is why preserving it early can decide the case.
On injury risk as a function of delta-V, see the NHTSA MAIS injury-probability curves. This is general information, not medical or legal advice.
