How police determine speed in a fatal crash

Local News

SIOUX FLALS, S.D. (KELO) — Figuring out how fast someone was driving in a fatal crash is a complicated process. There are a lot of factors to consider that involve physics, geometry and Newton’s laws on gravity.

“We use it pretty much on every fatality accident we have,” officer Chad Gillen said.

Officer Chad Gillen is a traffic officer with the Sioux Falls police department, but he’s also part of the accident reconstruction crew.

“A lot of it is based on Newtonian physics,” Gillen said.

In other words, police are looking at the mechanical events that took place leading up to and after the crash.

To find those, they can use 50 different formulas and equations based on different scenarios.

“You can use the tire marks on a skid to stop, you can use a formula to figure out that speed,” Gillen said.

They also use airborne equations; measuring how far a car vaulted in the air and where it landed.

“You can also use momentum to figure out speed based on, after the impact how far did the vehicles slide to a stop,” Gillen said.

Because a lot of today’s vehicles are built with computers, police also use something called Crash Data Retrieval.

“That’s where we use computers to go into the air bag control module and we can download the information off the car,” Gillen said.

That data tells you how fast the car was traveling, whether brakes were applied, and even how many degrees the steering wheel was turned.

But even that information Gillen says still has to be crossed checked mathematically using those equations and formulas.

If they come within a ballpark figure of about 5 mph, they know how fast someone was driving.

Gillen says it usually takes about two to three days for the reconstruction crew to determine a speed.

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