Skid Speed Calculator
Calculate vehicle speed from skid marks using the NHTSA friction formula for accident reconstruction.
Enter skid length, road surface, and braking efficiency.
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.