How to calculate speed from skid marks
Updated 2026 · 4 min read
A skid mark lets you calculate how fast a vehicle was going, and the arithmetic itself is straightforward. What decides the answer is not the formula: it is the friction coefficient you choose, and that is where reports are won and lost.
The formula
All the energy the vehicle carried is dissipated rubbing against the road surface, so it is enough to equate the two:
Where d is the length of the mark, μ the friction coefficient between tyre and road, p the gradient as a fraction (positive uphill) and vf the speed at the end of the segment, which is zero if the vehicle stopped there.
Friction is what governs the answer
Take a 30 metre skid to a stop, on dry level asphalt. Depending on which line of the CRASH II table you pick, you get this:
| Dry asphalt, below 48 km/h | published μ | Speed for 30 m |
|---|---|---|
| New and rough | 0.80 – 1.20 | 78 – 96 km/h |
| Travelled | 0.60 – 0.80 | 68 – 78 km/h |
| Traffic polished | 0.55 – 0.75 | 65 – 76 km/h |
The same skid gives 65 or 96 km/h depending on the state of the asphalt. If the speed limit was 80, the whole report turns on that choice — which is why a serious report states where its friction figure came from, rather than simply writing «dry asphalt: 0.8».
Friction falls as speed rises. On polished asphalt the same table gives 0.55–0.75 below 48 km/h and 0.45–0.65 above it: nearly a tenth of difference. In a long skid from high speed, using the low-speed value overestimates the initial speed.
The values are from Exhibit 9-5 of the CRASH II manual (McHenry, Calspan ZQ-5708-V-3, 1976), which distinguishes fourteen surfaces and, within each, wet from dry and above from below 48 km/h.
When the braking crosses different surfaces
Usually there is not a single segment: the vehicle starts braking on asphalt, crosses the central reservation and ends on soil. Each segment has its own friction and length, and the calculation is chained backwards: it starts from the final speed of the last segment and works in reverse to the beginning of the mark.
What the mark does not tell you
- ABS leaves no classic mark. A modern vehicle braking hard leaves faint, intermittent marks, not the continuous black stripe of a locked wheel. Measuring only what is clearly visible underestimates the braking.
- The mark starts late. Between the driver seeing the hazard and the wheels marking, the perception-reaction time and the brake system response time elapse. That distance is not in the mark.
- v₀ is the speed at the start of marking, not necessarily the speed the vehicle was travelling at before the driver reacted.
Module 09 of this calculator takes several segments with different friction and gradient, and returns the speed at the start of braking together with the time taken. For reaction time and the question of whether the accident was avoidable, see module 10.
Frequently asked questions
- What formula gives speed from a skid mark?
- v₀ = √(vf² + 2·(μ+p)·g·d), where d is the length of the mark, μ the friction coefficient, p the gradient and vf the speed at the end of the segment. If the vehicle stopped, vf is zero.
- How much does the friction coefficient change the result?
- Far more than any other input. A 30 metre skid on dry asphalt gives 78 to 96 km/h if the surface is new and rough, and 65 to 76 km/h if it is traffic polished. That is why the report must justify the value it uses.
- Can speed be calculated if the car had ABS?
- Yes, but not from a locked-wheel mark, because ABS does not leave one. It leaves faint intermittent marks whose real length has to be measured. Measuring only the clearly visible stretch puts the speed below the true value.
- Is the resulting speed the speed the driver was travelling at?
- It is the speed at the start of marking. Before that comes the distance covered during perception-reaction and brake system response, which the mark does not record and has to be added separately.
Sources
The friction coefficient is not picked by eye: the table the module uses comes from a published source.
- McHenry, R. R. (1976). User’s Manual for the CRASH Computer Program. Calspan ZQ-5708-V-3. Its Exhibit 9-5 lists fourteen surfaces with four pairs of friction values each.
How these methods compare against documented cases is detailed in How this calculator is validated.