What formula does this calculator use?

It uses the kinetic-energy / work-energy formula: v = √(2 × g × μ × d), where g = 9.80665 m/s², μ is the friction coefficient, and d is the skid length. This gives the minimum speed at the start of braking.

Why is this called a minimum speed estimate?

The formula assumes all kinetic energy is converted to heat through sliding friction. If the vehicle was still moving at the end of the skid marks (partial braking, ABS, or impact), the actual speed was higher than the estimate.

What friction coefficient should I use?

Typical values: dry asphalt 0.70–0.80, wet asphalt 0.50–0.60, dry concrete 0.70–0.80, wet concrete 0.45–0.55, packed gravel 0.55–0.65, ice 0.10–0.25. Use a drag sled or published road-test data for precise values.

Does this account for ABS brakes?

No. ABS prevents full lockup and leaves yaw marks rather than classic skid marks. This tool is designed for locked-wheel skid marks. ABS-equipped vehicles require different analysis methods.

Is the result admissible in court?

This tool provides an educational estimate only. Admissible accident reconstruction requires a qualified expert, calibrated instruments, an on-scene friction test, and peer-reviewed methodology. Do not use this output as sole legal evidence.

What if there are skid marks from multiple wheels?

Use the longest single continuous mark from any wheel. For staggered marks, investigators typically use the average of the four wheels or the longest mark, depending on jurisdiction standards.

Speed From Skid Marks

Name: Speed From Skid Marks
Availability: InStock
Author: Nham Vu

Estimate a vehicle's pre-braking speed from skid mark length and road surface friction — used in accident reconstruction.

Skid Mark Inputs

Skid Mark Length

Road Surface

Road Grade (%) — positive = uphill, negative = downhill

Leave 0 for level road. Uphill increases drag; downhill reduces it.

Enter skid mark length and select a surface to see the estimated speed.

Summary

Estimate a vehicle's pre-braking speed from skid mark length and road surface friction — used in accident reconstruction.

How it works

Enter the total skid mark length measured at the accident scene.
Select the unit (feet or meters) for the skid length.
Choose the road surface type or enter a custom friction coefficient.
Click Calculate — the tool applies the physics formula v = √(2·g·μ·d).
Review the estimated minimum pre-braking speed in mph and km/h.
Use the Notes field to record scene conditions for your report.

Use cases

Accident reconstruction by traffic investigators and insurance adjusters.
Estimating pre-collision speed when no data recorder (EDR) is available.
Driver-training demonstrations of braking distance vs. speed.
Legal proceedings requiring a physics-based speed estimate.
Engineering studies on road surface friction and stopping performance.
Educational exercises in forensic physics and automotive safety.

Frequently Asked Questions

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Last updated: 2026-05-29 · Reviewed by Nham Vu