# Skid Marks

I drove by a single car accident last night and the police were measuring the skid marks. I know this helps them determine the speed of the vehicle but my question is: does ABS breaks screw up their calculations? Does the police have a chart to determine the speed of a vehicle?

I’ve seen ABS breaks leave skidmarks but it wasn’t a continuous one like regular breaks. I was stopped at a light and somebody didn’t see the light change and you could see her standing on the breaks and wheels skid then moved then skid. She left a dotted skidmark all the way to the light.

Sorry, thought you were talking about the hershey squirt kind of skid marks.

That would certainly have made for a more interesting discussion.

D’oh

Do the police measure those, too?

In answer to the OP (I don’t know what the other guys are talking about, and I don’t wanna know!) it’s probably true that ABS screws up the measurements.

Skid marks are useful because the coefficient of friction for rubber sliding on pavement is, according to my old college professor, remarkably constant. If you know the weight of the car, or can estimate it pretty accurately, you can figure out how much force was being applied to stop the car. From that you can figure out the minimum speed the car had to be travelling to leave a skid mark that long.

So if someone swears they were only going 30 and they left a 40 mph skid mark, they are busted!

Since ABS is supposed to keep the wheels from skidding the skid marks (if they show up at all) are bound to be shorter than they would if someone just locked the wheels. Since the skid marks only indicate the minimum speed, a 40 mph ABS skid may now only be as long as a 30 mph non-ABS skid. Or, if the ABS worked perfectly, there would be no skid mark at all. So the evidence of driving too fast for conditions would be weaker or even non-existent.

## Bottom line – bad news if you’re the “Law and Order” guy, good news if you’re the perp.

“non sunt multiplicanda entia praeter necessitatem”
– William of Ockham

skid mark measurements are realy not very accurate. last year i hit an old man (in his car) while i was speeding–about 70 in a 45 zone-- and the cops measured the skid marks. they said that i was going 47, and gave the ticket to him for running a stop sign. i got off scott free.

eggo

eggo, that is NOTHING to be proud of. I could go into the scenario of how your speed and then untruths caused the old man’s family to decide the guy no longer was mentally capable of driving, shoved him into a nursing home, where he died a broken, lonely old soul, but I doubt even THAT would phase you.
(Ooops. Should I be banished to the BBQ pit?)

“There will always be somebody who’s never read a book who’ll know twice what you know.” - D.Duchovny