Delta-V is a property of a vehicle. Injury is a property of a person inside it. The two are related, and the relationship is loose enough that treating one as a proxy for the other is the most common overreach in this area — in both directions. A severity figure is an input to occupant analysis, not a conclusion about it. What connects them is the crash pulse, the occupant's position and restraint, and the interior surfaces reached during the fraction of a second that matters.
What delta-V describes
Delta-V is the change in a vehicle's velocity across an impact: a magnitude and a direction, the latter conventionally expressed as principal direction of force. It is a compact and genuinely useful severity descriptor, and it is what crush and momentum analysis are ultimately estimating.
What it omits is time. Two collisions with identical delta-V can distribute that velocity change over very different intervals, and the occupant does not experience the total — the occupant experiences the accelerations along the way.
The pulse carries the information
The crash pulse is the acceleration-time history of the occupant compartment: how quickly force built, how long it was sustained, whether it peaked once or several times. A long, progressive pulse from a deforming structure loads a restrained occupant very differently from a short, sharp pulse from a stiff or narrow engagement, even at the same delta-V.
This is why an underride, a pole impact, an impact into a rigid object, or a collision with a much heavier vehicle can produce occupant loading that the severity figure alone would not suggest.
Occupant motion is relative motion
The occupant continues at the pre-impact velocity while the compartment changes speed around them. Everything that follows is that relative motion being arrested — by belt webbing, by an airbag, by the interior, or by nothing until a hard surface is reached. Its direction is set by the principal direction of force, which is why frontal, oblique, side and rear impacts load the same person in the same seat differently.
Occupant position at impact — braced, turned, reclined, out of position, seat fore-aft setting — modifies all of it, and is usually inferred rather than known.
The restraint system is physical evidence
Belt hardware records loading. Webbing can show transfer marks, abrasion and stretch where it passed through the D-ring, latch plate and retractor under load; the retractor spool can show a witness pattern indicating webbing position when it locked; a fired pretensioner and a deployed load limiter are visible, physical states.
This answers the belt-use question far more directly than a switch state in a download, and it is destroyed routinely — by cutting occupants free, by repair, by salvage — before anyone asks.
Deployment is a threshold, not a severity gauge
An airbag fires when the restraint algorithm judges the sensed deceleration to warrant it for that impact type, within a window measured in milliseconds. Deployment therefore tells you a threshold was crossed. Non-deployment tells you it was not — which can be entirely correct behaviour in a low-severity impact, in a rear impact, or in an oblique or side event that the frontal algorithm was never intended to address.
Arguing from deployment to a specific severity, or from non-deployment to a minor collision, treats a binary decision as a continuous measurement.
What the deployed hardware shows
Beyond the fact of deployment: which modules fired and in what stage, whether the passenger system was suppressed by an occupant-classification decision, the fabric's contact and abrasion marks, and the residue pattern. Contact evidence on the interior — steering wheel rim deformation, header, pillar, glazing, seat back and head restraint condition — locates where occupants actually went.
Together these are more probative of occupant kinematics than any single severity number, and they exist only until the vehicle is repaired or scrapped.
Regulatory tests answer a different question
FMVSS 208 for frontal occupant protection and FMVSS 214 for side impact, along with NHTSA's consumer-information crash testing, evaluate a vehicle against defined test configurations using instrumented anthropomorphic test devices at prescribed speeds and angles.
That establishes whether a design met a standard under those conditions. It does not establish what happened to a particular person in a real collision at a different angle and speed, in a different seating position, with different stature, age and pre-existing condition. A vehicle can perform exactly as designed and an occupant can still be seriously injured.
Multiple events and rollover
Where a collision involves several impacts, each has its own delta-V and direction, and occupant loading must be attributed across the sequence rather than to a summed figure. A rollover is harder still: the loading is extended, rotational and repeated, and the injury mechanisms — roof intrusion, restraint slack, partial or complete ejection — are not the ones a single-impact severity number is built to describe.
The reconstruction sequence therefore has to be settled before injury causation can be discussed sensibly.
Where these opinions are challenged
Predictably: that a delta-V figure was mapped onto an injury threshold with no account of pulse, direction or occupant position; that belt use was asserted from a switch state while the hardware went uninspected; that non-deployment was offered as proof of a minor impact; or that a biomechanical opinion strayed into a clinical judgement it was not qualified to make.
Keeping the vehicle severity question and the occupant loading question visibly separate, and stating what connects them in this specific configuration, is what keeps both defensible.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.