Skid Speed Calculator: Speed from Skid Marks
Estimate how fast a vehicle was going from the length of its skid marks, using the standard friction formula.
Enter skid length, road surface, and braking.
Skid Speed Formula: Accident Reconstruction
When a vehicle brakes hard enough to lock its wheels, it leaves skid marks on the road. The length of those marks — combined with the road surface’s coefficient of friction — allows investigators to estimate how fast the vehicle was traveling.
NHTSA Formula (US Standard) S = √(30 × d × f) Where S = speed in mph, d = skid length in feet, f = friction coefficient (drag factor).
Physics-Based Formula (SI Units) v = √(2 × μ × g × d) Where v = speed in m/s, μ = friction coefficient, g = 9.81 m/s², d = skid distance in meters.
Braking Efficiency Real-world braking is rarely 100% efficient. ABS (Anti-lock Braking Systems) typically achieve 90–100% efficiency. Older drum brakes may only reach 70–80%. The formula adjusts as: v = √(2 × μ × g × d / e), where e = efficiency (0.0 to 1.0).
Friction Coefficients by Surface Different road surfaces resist sliding differently. Dry concrete (0.75) offers the most grip; glare ice (0.10) offers very little. The coefficient decreases significantly when wet — a fact critical to safe stopping distances.
Limitations Skid marks only appear when wheels are fully locked. Vehicles with ABS may leave partial or no marks, and ABS-equipped vehicles can actually stop in shorter distances than indicated by skid marks. Professional accident reconstruction requires additional factors: road grade, vehicle weight distribution, tire condition, environmental data, and witness testimony.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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