Can I determine the speed of a car from its skidmarks?

December 21, 2006 3:03 PM

A speeding driver just about rear-ended my delivery truck. I know how long her skidmarks are. Can I determine how fast she was driving?

Here's the story problem version of the scenario:

You own a box factory situated in the middle of the country.

Your delivery driver is returning from a trip. As every driver has done several times a day for twenty-one years, he idles up to the edge of the parking lot, angles across the entire (very low-traffic) road, and begins to back in.

Meanwhile, a woman in a Ford Focus comes flying around the corner (which is a quarter-mile down the road) and accelerates toward the the truck. The woman shows no signs of stopping. At the very last possible moment, she slams on her brakes. The tires smoke and squeal as the Ford Focus comes to rest three or four feet from the rear of the truck. After the truck has backed into the drive, the woman in the Ford Focus speeds away.

You happen to witness the whole incident because you were watching for the truck's return. You go out to measure the skidmarks. They're 45 feet, five inches long. (That's 13.84 meters to those so inclined.)

Question one. How fast was the Ford Focus traveling at the time the driver slammed on her brakes? (I own a Ford Focus, too. My owners manual indicates the car's mass is 3640lbs, or 1651kg.)

Question two. If an accident had occurred, who would be at fault? (This is in Oregon.) The truck driver followed all legally prescribed procedures except honking his horn — not that the woman could have heard it.

posted by jdroth to Grab Bag (17 answers total)

This claims to calculate speed from skid length, but it's going to be variable depending on road surface and tire type/condition. That tool says about 32 mph.
posted by Dipsomaniac at 3:07 PM on December 21, 2006

It also depends on the mass of the car, and the condition and tuning of the brakes.
posted by signal at 3:12 PM on December 21, 2006

Response by poster: Mass of the car, as stated in the question, is roughly 3640lbs (1650kg) plus passenger. Brake condition unknown, of course. Nice link, dipso.
posted by jdroth at 3:15 PM on December 21, 2006

Why condition of the brakes? If the wheel is skidding, the breaks must be locked, I'd think that is all you need to know.

I've got to think that ABS would make this kind of calculation much harder..
posted by Chuckles at 3:35 PM on December 21, 2006

Actually, the condition of the brake system may cause the skid marks to be deceptive. If the brakes fade excessively the wheels may actually unlock partway through stopping and so the length of the skid marks won't be an indicator of the car's speed.
posted by Dipsomaniac at 3:45 PM on December 21, 2006

I don't know if this can be done without heavy investigation and some heavy math. It's not just the condition of the tires; the condition of the road would be a factor as well. Concrete? Asphalt? Dry? Rocky? Basically any factors that contribute towards finding the coefficient of friction between the surfaces.

I think in forensic analysis, they not only measure the length of the mark, but the amount of rubber deposited. And in most accidents, they would find how far pieces (of glass, plastic) flew from the site of impact. Without impact, I think you need to find info on her brakes as well, and how well the ABS worked at the time.

Besides, 30mph doesn't seem fast enough to account for the smoking and squealing that you described.
posted by krippledkonscious at 3:49 PM on December 21, 2006

I got about 30mph from the formulas too, using ballpark estimates.
posted by unSane at 4:32 PM on December 21, 2006

By the way, the estimates assume quite efficient braking. If her tires were bald or the weight transfer to the front of the car was excessive the speed might have been lower.
posted by unSane at 4:33 PM on December 21, 2006

Response by poster: Based on your links and formulae, I've done my own calculations and come out in the neighborhood of 30mph, too. Too bad "rapid slowing" doesn't leave skidmarks. I'm dying to know just how fast the driver was going say five seconds before the braking occurred. She was flying.

Thanks!
posted by jdroth at 4:55 PM on December 21, 2006

No CCTV on the street or from any adjacent buildings you could watch back?
posted by chrissyboy at 5:46 PM on December 21, 2006

From what I gather online, the curb weight of the Focus is more like 2700lb. Perhaps the 3600 figure you're quoting is the GVW, or gross vehicle weight rating, which generally includes as many 180lb passengers as can fit, plus some cargo. Try adding the weight of one woman (150lb?) to the 2700 and work with 2850 when you're doing the math.
posted by knave at 6:00 PM on December 21, 2006

The problem with this situation is there really are too many variables.

• Were her tires worn out, or in good shape?
• Was the road wet?
• Was the road cold?
• Did she gradually apply the brakes at first, bleeding off maybe 10mph before locking up?
• Does the Focus have a crappy suspension leading to almost no weight over the rear wheels while braking?
• Was the road surface uneven or smooth?
• Was the car equipped with ABS? (choppy skid pattern?)
• Did she unlock the tires at some point and then slow, without skidding, to a stop?

posted by knave at 6:04 PM on December 21, 2006

The problem with this situation is there really are too many variables.

Real police investigators can crunch through all of those variables and arrive at a speed, with an acceptable margin of error. This happens everyday in every courtroom in the land.
posted by frogan at 6:36 PM on December 21, 2006

Would there not be a problem because whatever speed she was doing when she hit the van would have rapidly reduced to zero without corresponding skidmarks, ie whether it was 1mph, 5 mph or 15mph? Even with all the variables accounted for aren't you just getting an idea of the speed reduction between the start of braking and the impact rather than the speed at which braking began?
posted by biffa at 2:48 AM on December 22, 2006

She didn't hit the truck, biffa. "The tires smoke and squeal as the Ford Focus comes to rest three or four feet from the rear of the truck.
posted by DevilsAdvocate at 6:45 AM on December 22, 2006

Jesus, I've been like a cabbage all week. Time to give up and get in some last minute xmas shopping I think.
posted by biffa at 7:09 AM on December 22, 2006

When I lived in NC, I seem to recall someone getting cited for 'failure to avoid' [an accident], when it was clear that avoidance was possible, even though the other party was also in violation of a law at the time.

There's some issue of contribution, and probably some civil remedy available after a collision, even absent any citation.
posted by FauxScot at 7:55 AM on December 22, 2006