How do you prove an accident was unavoidable?

By comparing the distance the driver needed to perceive the hazard, react and brake with the distance actually available to the point of impact. If the required distance is greater than the available one, the accident was unavoidable at that speed.

What reaction time is applied in an expert report?

There is no universal value. Figures between 0.75 and 1.5 seconds are usually handled depending on whether the hazard was expected or appeared by surprise. At 50 km/h that difference is more than ten metres and often decides the verdict, so the report must declare which value it uses and why.

If the driver was speeding, is the accident always their fault?

Not necessarily. You also have to calculate whether it would have been avoidable at the legal speed limit. If it was unavoidable at both speeds, the excess did not cause the accident, although it remains an offence.

How far does a car travel before the brakes act?

At 50 km/h with one second of reaction, almost 14 metres. On dry asphalt the braking adds about 12 more, so more than half the stopping distance is consumed before the brakes take effect.

Could the accident have been avoided?

Updated 2026 · 4 min read

It is the question that decides many proceedings, and it has a calculable answer. The distance the driver needed to perceive the hazard, react and stop is compared with the distance actually available. If the first is greater, the accident was unavoidable at that speed.

The two distances

Total stopping distance has two terms that behave very differently:

d_stop = v · t_r + v² / ( 2 · (μ + p) · g )

The first is what is covered while the driver has not yet done anything: perceiving, deciding and moving the foot to the brake. It grows in direct proportion to speed. The second is the braking itself, and it grows with the square of speed.

Why the difference matters

At 50 km/h with one second of reaction, the vehicle covers almost 14 metres before braking begins. On dry asphalt the braking adds about 12 more. That is: more than half the stopping distance is consumed before the brakes have acted. At 100 km/h the proportion reverses, because the braking has quadrupled while the reaction has only doubled.

Reaction time is the most disputed input

There is no universal value. It depends on whether the hazard was expected or appeared by surprise, on visibility, and on the age and state of the driver. The values usually handled run from 0.75 to 1.5 seconds, and at 50 km/h the difference between those extremes is more than ten metres — frequently the difference between avoidable and unavoidable.

A serious report therefore does not fix a reaction time: it declares which one it uses, why, and what would happen to the verdict if it moved within its reasonable range.

The question the court is really asking

Often it is not «was it avoidable?» but «would it have been avoidable at the legal speed limit?». Those are two different calculations, and the second is the one that separates liability. It is perfectly possible for the accident to have been unavoidable both at the actual speed and at the legal one — in which case the excess speed was not the cause of the impact, even though it remains an offence.

There is a third figure that usually closes the discussion: the maximum speed at which the driver would in fact have stopped in time. It follows from the same equation and gives a concrete value, not an opinion.

The case of the truck pulling away

A frequent variant: a heavy vehicle pulling out of a stop line or finishing clearing a junction. There the distance alone is not enough — you need to know how long the truck takes to clear and where the other vehicle was at that instant. It is resolved with measured pull-away times by vehicle type and load.

Module 10 compares the actual speed with the legal one, gives a clear verdict —avoidable or unavoidable— and the maximum speed that would have allowed a stop, with an optional block for the heavy vehicle pulling away.

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Frequently asked questions

How do you prove an accident was unavoidable?: By comparing the distance the driver needed to perceive the hazard, react and brake with the distance actually available to the point of impact. If the required distance is greater than the available one, the accident was unavoidable at that speed.
What reaction time is applied in an expert report?: There is no universal value. Figures between 0.75 and 1.5 seconds are usually handled depending on whether the hazard was expected or appeared by surprise. At 50 km/h that difference is more than ten metres and often decides the verdict, so the report must declare which value it uses and why.
If the driver was speeding, is the accident always their fault?: Not necessarily. You also have to calculate whether it would have been avoidable at the legal speed limit. If it was unavoidable at both speeds, the excess did not cause the accident, although it remains an offence.
How far does a car travel before the brakes act?: At 50 km/h with one second of reaction, almost 14 metres. On dry asphalt the braking adds about 12 more, so more than half the stopping distance is consumed before the brakes take effect.

Sources

The acceleration profiles used to decide whether the accident was avoidable:

Kodsi, S. & Muttart, J. (2010). «Modeling Passenger Vehicle Acceleration Profiles from Naturalistic Observations and Driver Testing at Two-way-stop Controlled Intersections». SAE 2010-01-0062. Observation of 244 drivers at eleven intersections.
For heavy vehicles, SAE 2020-01-0882, over 104 trucks. It also warns that a truck is tracked at a point roughly above the cab, not at the front as with a car.
Fricke, L. B. Constant acceleration of 0.15 g, used as a slow-manoeuvre reference.

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