After a car accident, you can determine about how fast a car was going when it hit the brakes by the length of the skid mark left by the tires. Often times, law enforcement agencies and insurance companies will calculate the speed the car was going to determine liability and potential law violations. To calculate how fast the car was going, determine the drag factor, braking efficiency and the distance of the skid marks.
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Measure the distance of the skid marks in feet and inches. If there are multiple skid marks, measure all of them and take the average.

Convert the measurement to a decimal by dividing the number of inches by 12 and adding the result to the number of feet. For example, if the skid mark was 120 feet 6 inches, you would divide 6 by 12 to get 0.5 and then add 120 to get 120.5 feet.

Estimate the drag factor of the road that you were traveling on. The drag factor represents how quickly a car slows down on a given surface. Asphalt roads usually range from 0.50 to 0.90, gravel from 0.40 to 0.80, ice form 0.10 to 0.25 and snow from 0.10 to 0.55. Usually skid tests will have to be performed after the accident to determine the exact drag factor.

Determine the braking efficiency based on how many skid marks are left. If a car leaves four skid marks the breaking efficiency is 100 percent. For each front wheel that is not skidding, subtract 20 percent. For each rear wheel that does not leave a skid mark, subtract 30 percent.

Calculate the skid speed by multiplying the drag factor times the braking efficiency times the skid distance times 30 and taking the square root of the result. For example, if the car skidded 120.5 feet on a road with a drag factor of 0.5 and left four skid marks, the car was going about 42.51 mph.
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