The Third Collision

Every crash is three collisions. The first one is in the photographs. The third one is in the autopsy.

The first collision is the car against whatever it hit. The front of the car folds, by design, over a foot or two of steel, and that folding is what turns sixty miles an hour into zero without turning the cabin into the thing that stops. The second collision is the body against the car. The car has stopped; the person inside has not. The belt catches the pelvis and the shoulder, the airbag catches the face, and if neither is there the dash and the steering wheel do it instead. The skeleton stops.

The third collision is the one inside. The organs are not bolted to the skeleton. They hang. The heart and the great vessels above it are mobile in the chest; the brain floats in fluid inside the skull; the liver and spleen sit on their stalks. When the skeleton stops, each of those keeps going at the speed the body had a hundredth of a second earlier, until it is stopped by whatever tethers it. The tether is where the injury is.

Take the aorta, the largest artery, rising out of the heart and arching down the back of the chest. The arch and the heart it comes from are free to move. The descending part is fixed to the spine. The place where the moving part meets the fixed part is called the isthmus, and it is tied down there by a small ligament left over from before birth. In a hard deceleration the arch swings forward and the isthmus does not, and the ligament, in the words of the medical reference, amplifies the shearing forces at exactly that point. The vessel tears. About eighty percent of the people it happens to die before they reach a hospital, and about four out of five of these injuries come from a motor vehicle crash. Nothing has to enter the chest. Nothing has to break the ribs. The person can look untouched.

The brain does the same thing on a smaller scale. When the head snaps and turns, the brain rotates inside the skull a beat behind it, and the long fibers that carry its signals are stretched and torn where soft matter meets firmer matter, in the bridge between the hemispheres and in the stem. That is diffuse axonal injury, and the reference on it is blunt: no impact is required, and the principal force is the angular acceleration during a high-speed deceleration. Road crashes account for about eighty percent of it in adults. The first CT scan is usually clean. The person is not.

Understand what this does to the defense story, because the story is always the same and always comes with photographs. Look at the bumper. Look how little damage there is. A bumper is a picture of the first collision. It says nothing about the third, because the third collision does not scale with how bent the car is. It scales with how fast the body’s speed changed and how quickly, the number engineers call delta-V. A car that crumpled beautifully, that did its job and kept the cabin intact, can still deliver a change of velocity to the person inside that the aorta cannot survive. A car that barely dented can deliver, to a small person in a light car hit by a heavy one, a change of velocity that a brain does not forgive.

So build the third collision the way you would build any other fact. The reconstructionist gives you the delta-V from the crush and the data recorder. The emergency physician’s note gives you the mechanism as medicine recorded it in the first hour, before anyone had a theory. The chest film gives you the widened shadow in the middle of the chest that a torn aorta leaves. The MRI on day nine gives you what the CT on day one could not see. Put those in order for the jury and the bumper photograph becomes what it always was, a picture of the wrong collision.

On the aorta, see Traumatic Aortic Injuries (Harper, Collier, and Slesinger, StatPearls, National Library of Medicine, updated March 2024), on the mobile arch, the fixed isthmus, the ligament that concentrates the shear, and the figures for prehospital death and motor vehicle crashes. On the brain, see Diffuse Axonal Injury (Mesfin, Gupta, Hays Shapshak, and Margetis, StatPearls, updated July 2025), on shear at the gray-white junction, the corpus callosum and brainstem, and the share of cases from road crashes. The number that governs all three collisions is explained in Delta-V. General information, not medical or legal advice.