Enter your Length of Skid Marks, Friction Coefficient, Roadway Surface Type, Road Slope, and Braking Efficiency into the Speed from Skid Marks Calculator to find the vehicle's Estimated Speed at the time of braking — plus Speed in km/h and the Adjusted Friction value accounting for slope and surface conditions.
Estimated Speed
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Speed (km/h)
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Adjusted Friction
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The accuracy depends on precise measurement of skid marks and correct friction coefficient estimation. Professional accident reconstructionists typically achieve accuracy within 10-15% when all variables are properly measured.
Use 0.7-0.8 for dry asphalt, 0.6-0.7 for dry concrete, 0.4-0.5 for wet asphalt, and 0.3-0.4 for wet concrete. Very slippery conditions (ice, oil) may be as low as 0.05.
Uphill slopes increase the calculated speed (vehicle was going faster to create same skid length), while downhill slopes decrease it. The slope is entered as a percentage where positive values are uphill and negative values are downhill.
Braking efficiency represents how well the vehicle's brakes were working. 100% means optimal braking, while lower values account for worn brakes, brake fade, or partial brake failure. Poor braking efficiency results in longer skid marks at the same speed.
Yes, the physics formula applies to all vehicles. However, factors like tire condition, vehicle weight distribution, and ABS systems can affect actual braking performance and should be considered in professional accident reconstruction.
Use the longest continuous skid mark for calculation. For curved skid marks, measure the actual path length. Multiple skid marks may indicate different phases of braking and require separate analysis.