The case of the rare Ferrari Enzo sports car that crashed and split in two on the Pacific Coast Highway moved into the courts on Monday. Prosecutors charged the man who seems to have been at the wheel with embezzlement, grand theft, drunk driving, and weapons possession. According to the Los Angeles County sheriff's office, the car was traveling at 162 mph at the time of the accident. How do investigators figure out the speed of an auto collision?
They look at where the car and all its pieces end up. (Bits of the Ferrari were scattered over 1,200 feet of highway.) If investigators can figure out the direction and distance each piece traveled from the site of the impact, they can infer the speed at which the car was going when it crashed.
In order to perform this kind of analysis, you first need to figure out where exactly the impact occurred. Marks and debris on the highway—like gouged pavement, broken glass, or oil stains—can give you a good idea of where the accident began and what happened to the car in its aftermath. Then, you need to find out the "coefficient of friction" of the surface on which the accident took place. This tells you how quickly a car (or a piece of car) would skid to a stop on that surface—whether it's pavement, dirt, or grass.
Given these data, you can figure out the velocity of the car as it came out of the collision—which can then help you to figure out its velocity going in. In general, you know that both momentum and energy will be conserved over the course of the accident. For example, the momentum of the speeding Ferrari before the crash would translate into a momentum for each of its two halves, and for the utility pole that it crashed into (and knocked upside down). The same goes for energy: If you know the amount of energy it takes to snap a utility pole in two, or the amount it takes to break a Ferrari in half, you can work backward to figure out how much energy the Ferrari must have had just before the impact.
You can also figure out how fast a car was going by looking at how much of it got crushed on impact. Manufacturers and insurance companies run tests to see what happens to a car when it hits a barrier at various speeds. This gives crash investigators a frame of reference for evaluating crush damage at the scene of an accident.
Most cars now have something like a black box that records its speed during the few seconds before an accident. (The "event data recorder" uses this information to deploy airbags when necessary.) Some manufacturers, like GM and Ford, make it easy for investigators to download the information from these boxes after an accident. Foreign car manufacturers are more likely to keep the event data to themselves. But you can't just go with the number from the recorder, since some of them only keep track of the car's speed along its length, as opposed to its speed going sideways. Collision analysts generally look at the momentum, energy, and crush damage, and then compare their speed estimates to the number coming out of the black box.
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Explainer thanks Chris Kauderer of Rudy Degger & Associates and Dave McClenahan of the Institute of Police Technology and Management. Thanks to reader Gideon Brower for asking the question.