The file arrives with two photographs of a rear bumper, a repair estimate for a few hundred pounds and a claim for a neck injury. One side says the impact was trivial and could not have hurt anyone. The other says the claimant was injured and the photographs prove nothing. Both are partly right, and the engineering explains why.
What a bumper is designed to do
A modern bumper is built to absorb a low-speed impact without visible damage. UN Regulation No. 42, which governs front and rear protective devices, is tested with pendulum and barrier impacts at low speed, and the design intent is that the vehicle’s safety systems and bodywork survive them. Behind the painted cover sits an energy absorber, often a foam block or a crushable plastic structure, and behind that a beam. Those parts can be compressed, recover their shape, and leave a cover that photographs as nearly undamaged.
The consequence is that the energy of a low-speed impact is, by design, hidden from the camera. A bumper that looks untouched may have transmitted a meaningful pulse to the vehicle, and a bumper that looks scuffed may have done no more than exchange paint. The photograph records the cosmetic outcome. It does not record the energy.
Delta-V: the quantity that matters
The measure that relates an impact to what the occupants experienced is delta-V, the change in the vehicle’s velocity during the impact. For the struck vehicle in a rear-end collision, delta-V depends on the closing speed, the ratio of the two masses, and how much of the impact energy was absorbed by deformation rather than returned as rebound. A light car struck by a heavy one at a given closing speed experiences a larger delta-V than a heavy car struck by a light one. The same scuffed bumper can therefore sit in front of quite different occupant loadings.
Human-volunteer studies of low-speed rear impacts, including the SAE series by Szabo and by Siegmund and their colleagues in the 1990s, measured occupant head and neck motion at delta-Vs of a few miles per hour and recorded the kinematics in detail. That literature is the basis on which a medical expert can relate a calculated delta-V to the loading an occupant was exposed to. It is also the literature on which the limits of that relationship rest: the volunteers were braced, expecting the impact, and seated in known postures, and the studies say so.
What the engineering can and cannot say
An engineer can, from the vehicles’ masses, the damage, the stiffness of the structures involved and any available data, estimate the closing speed and the delta-V of the struck vehicle as a range. The engineer can say whether the damage to each vehicle is consistent with the other and with the accounts. The engineer can say how that delta-V compares with the delta-Vs at which the published volunteer studies were run, and leave the significance of that comparison to the medical expert.
What the engineer cannot say is whether a particular person was injured. That is a medical question, and the honest engineering report hands it over expressly, with the delta-V range and the caveats attached. The courts have been clear that the two disciplines must not be blurred.
How the courts handle these claims
In Kearsley v Klarfeld the Court of Appeal considered how a defendant who contends that the impact could not have caused injury should plead the case, and held that it is for the defendant to set out the contention clearly so that the claimant knows the point being taken. In Casey v Cartwright the Court gave guidance on when permission should be given for expert evidence on the causation issue in low-velocity impact claims: the defendant should notify the court and the other parties within a short time of the defence that the issue is to be raised, and the court will then consider, with proportionality in mind, whether expert evidence is reasonably required under rule 35.1.
The practical effect is that engineering evidence in these cases is admitted only where it earns its place. A report that simply repeats “the damage was minor” adds nothing to the photographs. A report that derives a delta-V range with its reasoning shown, deals candidly with the uncertainty in the inputs, and stops at the boundary of the medical question gives the court something it can actually use.
For those instructing
Three things make the engineering in a low-velocity impact claim more reliable than it would otherwise be. Obtain the repair estimates and photographs for both vehicles, not one. Ask early whether either vehicle has an event data recorder that captured the impact, because recorded delta-V is worth more than any calculation. And instruct the engineer to answer the engineering question only, leaving the injury question to the medical expert with the engineering report in front of them.