Speed from tire marks
Skid Mark Speed Calculator
Estimate the minimum speed at the beginning of a skid or slide using measured distance, surface drag factor, and the conditions that affected braking.
Level-road formula
For a level surface and a vehicle that slides to rest, the familiar U.S. customary relationship is:
The metric equivalent uses the corresponding unit conversion. Grade, multiple surfaces, incomplete braking, impact speed, or rolling after the mark require a more specific energy calculation.
Evidence to document
- Beginning and end of each tire mark and the distance measurement method.
- Mark classification: skid, scuff, yaw, imprint, or another type.
- A measured drag factor and the test conditions.
- Roadway grade, braking contribution, and whether the vehicle stopped at the mark's end.
How to interpret the result
A slide-to-stop result is normally a minimum speed at the beginning of the measured energy-loss segment. It is not automatically the vehicle's pre-event speed.
Record uncertainty and use an independent method where possible. The calculator shows substituted values and arithmetic so another investigator can reproduce the result.
Related tools
Multiple skid segments · Drag factor from a test skid · Critical speed yaw