When you slam on the brakes in your car, the tires lock, causing them to skid across the road. The length of the skid can help determine how fast you were going when you hit the brakes. This type of analysis can determine who was at fault in an accident or if traffic violations have occurred. To estimate the speed at the time the brakes were applied, you need to measure the skid marks, estimate the drag coefficient and determine the braking efficiency.
Things You'll Need
 Measuring tape
 Calculator

Use the measuring tape to determine how far the car skidded. If there are multiple skid marks, measure them all. If there was only one mark, skip to step 3.

Take the average of the distances by adding the distance of each skid mark and dividing by the number of skid marks.

Determine the braking efficiency based on the number of skid marks found. Each back tire skid mark contributes 30 percent while each front tire contributes 20 percent. For example, if there were two skid marks from the back wheels and one skid mark from a front wheel, the braking efficiency would be 80 percent.

Estimate the drag coefficient based on the surface on which the car was driving. The drag coefficient represents how much the road contributes to stopping the car, with lower drag coefficients representing slicker roads. For example, asphalt generally has a drag coefficient of 0.5 to 0.9 and gravel drag coefficients come in between 0.4 and 0.8. Ice brings it down to between 0.1 and 0.25 and snow drops the coefficient to somewhere between 0.1 and 0.55. If you need an exact number, you will have to do skid testing on the road.

Calculate the estimated speed of the vehicle by taking the square root of the product of 30 times the average skid distance times the braking efficiency times the drag coefficient. For example, if the skid average was 140 feet, the braking efficiency was 0.8 and the drag coefficient was 0.6, the estimated speed would be 44.90 mph.
Tips & Warnings
 Measure the distance of the car in feet. If you use a different unit of measure you will get an incorrect speed.
 Skid marks from a tire with antilock brakes will leave a dashed skid mark but you should still measure from the start of the dashes.
References
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