What is EES in a traffic accident?

It is the speed at which a vehicle would have to strike a rigid barrier to suffer the same permanent deformation it shows in the accident. It measures the energy absorbed by the bodywork, not the speed it was travelling at.

Is EES the same as impact speed?

No. EES measures the energy a vehicle absorbed. The impact speed is obtained afterwards, by sharing that energy between the two vehicles using conservation of momentum.

Why does the method not work below 16 km/h?

Because it assumes the deformation is permanent and the rebound negligible. At low speed much of the deformation recovers, and what is measured stops representing the energy dissipated. The PC-Crash manual sets the same limit on its equivalent module.

Where do the A and B stiffness coefficients come from?

From instrumented barrier tests on real vehicles, not from estimates. The result therefore depends on the case vehicle resembling those tested, and the report should declare which group the stiffness was taken from.

Can you tell the speed from the damage to a car?

Updated 2026 · 4 min read

Yes, within limits. The method is called CRASH3, it has forty years of use behind it and it is the same one the commercial reconstruction packages employ. But there is a speed below which it stops being applicable, and it is worth knowing where that is before relying on the number.

What EES is

EES (Energy Equivalent Speed) is the speed at which that same vehicle would have to strike a rigid immovable barrier to end up with the permanent deformation it shows in the accident.

It is not the impact speed, and confusing the two is a frequent error. It is a measure of the energy the bodywork absorbed, expressed in speed units because that is convenient. The impact speed comes afterwards, by sharing that energy between the two vehicles using conservation of momentum.

What has to be measured

Crush measurements are taken along the damage —two, four or six points, depending on how much detail the vehicle allows— and from them the deformation profile is reconstructed. The energy follows from two coefficients specific to each vehicle, A and B, which are not estimated by eye: they come from instrumented, measured barrier tests.

The correction that gets forgotten

If the force did not come in head-on but obliquely, part of the work went into sliding and the real energy is greater than the measured crush suggests. The method corrects for the direction of force (PDOF), and in a 30° impact the factor exceeds 33%. A report that measures the crush carefully but ignores the direction of force falls short.

Where it stops working

This is the important part and the one most often overlooked:

Below about 16 km/h CRASH3 is not applicable

The method assumes the deformation is permanent and the rebound negligible. At low speed that stops being true: much of the deformation recovers, the bumper returns to its place and what is measured no longer represents the energy that was dissipated. This is not an opinion: the PC-Crash manual sets the same limit on its own equivalent module. This calculator warns when the result falls below it.

There is a second case in which the number is worthless, and no package warns about it: underride beneath a truck. If the vehicle passes under the trailer body, the crush measured at the front does not represent what the structure absorbed, and the resulting EES falls well below reality. That case has to be reconstructed by another route.

How accurate it is

It depends on the regime. In low-severity impacts the method reproduces the ΔV with a mean error of around 10%, and across the whole sample the median error rises to 15%. The specific figures, with sample sizes and limitations, are in how this calculator is validated.

What cannot be done is to present the result as an exact value. An EES is always a range, and the report should declare it.

Module 05 calculates the impact speed against a fixed object from the measured crush, and module 07 does the same with the damage to both vehicles in a frontal collision, without needing any motion data.

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Frequently asked questions

What is EES in a traffic accident?: It is the speed at which a vehicle would have to strike a rigid barrier to suffer the same permanent deformation it shows in the accident. It measures the energy absorbed by the bodywork, not the speed it was travelling at.
Is EES the same as impact speed?: No. EES measures the energy a vehicle absorbed. The impact speed is obtained afterwards, by sharing that energy between the two vehicles using conservation of momentum.
Why does the method not work below 16 km/h?: Because it assumes the deformation is permanent and the rebound negligible. At low speed much of the deformation recovers, and what is measured stops representing the energy dissipated. The PC-Crash manual sets the same limit on its equivalent module.
Where do the A and B stiffness coefficients come from?: From instrumented barrier tests on real vehicles, not from estimates. The result therefore depends on the case vehicle resembling those tested, and the report should declare which group the stiffness was taken from.

Sources

The CRASH3 method, the stiffness coefficients and the tests it has been checked against:

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.
Wood, D., Doody, M. & Mooney, S. (1993). SAE 930894. Model for impact against a narrow object.
Malmsbury, R. et al. (1994). «Damage And/Or Impact Absorber (Isolator) Movements Observed In Low Speed Crash Tests Involving Ford Escorts». SAE 940912.
Szabo, T. et al. (1992). «Dynamics of Low Speed Crash Tests with Energy Absorbing Bumpers». SAE 921573.
Navin, M. et al. (1987). «Crash III And Canadian Test Data». SAE 870499. CRASH III checked against Canadian test data.
The A and B stiffness coefficients have been rebuilt from the raw measurements of NHTSA’s rigid-barrier frontal tests.

How these methods compare against documented cases is detailed in How this calculator is validated.