Skid Speed Calculator: Speed from Skid Marks

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, lets an investigator 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.

Those two are the same equation wearing different units. The 30 in the US version is a rounded conversion constant; carried out exactly it is 29.92, so the two formulas disagree by about 0.13%. This calculator runs the US form and converts, which keeps the mph, km/h and m/s figures consistent with each other instead of having them drift apart in the last digit.

Braking Efficiency Real-world braking is rarely 100% efficient. Anti-lock brakes (ABS) typically achieve 90 to 100%. Older drum brakes may only reach 70 to 80%. The formula becomes v = √(2 × μ × g × d × e), where e is the efficiency between 0 and 1. Lower efficiency means less deceleration, so the same skid mark corresponds to a lower starting speed.

Road grade A slope changes the answer more than most people expect, and it is the one factor a real reconstruction never leaves out. Braking uphill, gravity helps you stop, so a given skid length must have started from a higher speed. Braking downhill, gravity fights the brakes and the same marks mean a lower speed.

The drag factor becomes f = (μ × e) + G, where G is the grade as a decimal and is positive uphill. On a 6% downgrade with wet asphalt at μ = 0.45, the effective drag factor drops to 0.39, which cuts the speed estimate by about 7%. On glare ice the same 6% downgrade drops it from 0.10 to 0.04 and takes more than a third off the answer.

Friction Coefficients by Surface Different road surfaces resist sliding differently. Dry concrete at 0.75 offers the most grip and glare ice at 0.10 offers very little. Every one of these drops sharply once the surface is wet, which is the whole reason stopping distances are quoted for dry conditions and then hedged.

This is a minimum speed, not the speed The formula only accounts for the energy the tires scrubbed off during the visible skid. It says nothing about speed lost before the wheels locked, during the driver’s reaction, in a collision at the end, or through an impact with anything the vehicle struck. So the number it produces is a floor. The vehicle was going at least that fast, and quite possibly faster.

Limitations Skid marks only appear when wheels are fully locked. Vehicles with ABS may leave partial marks or none at all, and an ABS-equipped car can stop shorter than its marks suggest. Professional accident reconstruction also needs vehicle weight distribution, tire condition, environmental data, and witness testimony.