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# How to Determine the Speed of a Car by the Length of a Skid Mark

Updated April 17, 2017

Harsh braking in a motor car causes the tires to lock, resulting in the car skidding. The skid marks created during this process can be used to estimate the speed the car was travelling when the brakes were pressed. This can then be used to help determine who was to blame in a traffic accident or if any traffic violations occurred. To determine the speed of the car you need to complete a formula based on the length of the skid mark, the braking efficiency of the car and drag coefficient of the road.

Measure each individual skid mark using a measuring tape. Record the measurement for each mark working in feet. If you are working out the speed based on a single skid mark then skip step 2.

Work out the average length of the skid marks by adding the distances together before dividing by the number of skid marks.

Calculate the braking efficiency of the vehicle. Use the skid marks found to work out the braking efficiency. Each mark left by a rear tire counts for 30 per cent while each front wheel mark counts for 20 per cent. For example, if you find one rear wheel skid mark and two front wheel skid marks then the braking efficiency would equal 70 per cent.

Find an estimate for the drag coefficient of the road surface. This represents the roads contribution to stopping the car. For example the average drag coefficient for asphalt is between 0.5 and 0.9 while an icy surface has a much lower coefficient of between 0.1 and 0.25.

Determine the speed at which the car was travelling. To calculate an estimate multiply the average skid distance by 30 then multiply this figure by the braking efficiency then multiply this final figure by the drag coefficient. Once you have completed the multiplications work out the square root of this number to find the estimated speed. For example if the average skid was 150 feet, the braking efficiency was 0.6 and the drag coefficient was 0.5 then the estimated speed would be 36.74mph.

#### Things You'll Need

• Measuring tape
• Calculator