Category Archives: Cars

Forensics on the Road

Last fall I took a course called Motor Vehicle Accident Investigation, and it was one of the most fun and informative classes of my grad school career. Every chance he got, our instructor took us out of the classroom to do actual field work. One such afternoon we conducted skid tests, in order to practice taking measurements and analyzing evidence.

In the video below, you can see what one such skid test looked like. Yes, that’s me in the passenger seat! It was a BLAST 🙂
What you should note is that my classmates are placing chalk marks at the beginning of the skid at what is called the “erasuremark” – it may seem like there is no skid where they are marking, but if you look closer you can see a faint outline of where the heat produced by the skidding tires hasn’t built up enough yet to lay down sufficient rubber. Measurements need to be taken starting with the beginning of the erasuremark.

In this next video, we explored what happens when a car skids with its wheels turned… (pay no attention to the noise in the background; we were at a police training facility so random gun shots and noise were to be expected)

NO, you cannot turn out of a skid when you’re on a dry road surface, so don’t believe anyone who says they can. And like my professor, Roger, states, note the difference in the appearance of the skid marks: it’s very hard to pick out any tread or marks from the ribs of the tires. All of this gives investigators clues to what happened.

There’s a lot that goes into this besides stretching a measuring tape and laying down evidence cards. There’s actual MATH involved. I hate math. But Roger made this really easy. Suppose you needed to figure out the speed of a vehicle involved in a crash using its skid marks. I will use the example from the skid test in video #1 above. There’s a formula: S (speed) = the square root of 30 (a constant) X “d” (distance in feet) X “f” (drag factor). Bear with me here… I promise it won’t be bad. To get the drag factor, we used an accelerometer in the test car. But it can be obtained mathematically as well (I will spare you the calculations). And what you can’t see in the video is the student with the radar gun sitting off-camera to obtain the speed of the vehicle. The skid measured 81.2 feet. The square root of 30×81.2x.57 (our drag factor) = 37 miles per hour. The radar speed obtained was 35 mph! Pretty close! So if we hadn’t had the radar gun we could have determined speed by basic math! Cool, huh? Yes, I’m a geek. There are also formulas for calculating time, distance, velocity at acceleration, velocity at deceleration, etc., etc. And if you’d like me to explain drag factor and the formula for determining it, I would be happy to. Just know that you are a bigger geek than me, and I think you’re great. 🙂

More vehicle accident forensics to come later.