Speed from Skid Marks Calculator

When a vehicle brakes hard enough to leave marks on the road, those skid marks contain clues about how fast it was traveling — the Speed from Skid Marks Calculator uses forensic physics to estimate that speed. Enter the length of skid marks, roadway surface type, friction coefficient, road slope, and braking efficiency to get the estimated speed in mph and km/h. The calculator also returns the adjusted friction coefficient used in the calculation, accounting for slope and braking system performance.

ft

Measured distance from start to end of skid marks

0.05 = Very slippery, 0.7 = Normal dry, 0.9 = Ideal braking

%

Use 0% for flat road, negative for downhill, positive for uphill

%

Vehicle braking system effectiveness (100% = optimal)

Results

Estimated Speed

--

Speed (km/h)

--

Adjusted Friction

--

More Automotive Tools

Frequently Asked Questions

How accurate is the speed calculation from skid marks?

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.

What friction coefficient should I use for different road conditions?

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.

How does road slope affect the speed calculation?

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.

What is braking efficiency and how does it affect results?

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.

Can this calculator be used for all types of vehicles?

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.

What if there are multiple skid marks or they're not straight?

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.