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The speed change, Delta V, and the PDOF, principal direction of force, on a vehicle in an accident can be taken together to form a useful measure of accident severity. Many studies have correlated the combined effect of these with the statistical probability of an injury of a certain type and severity according to the AIS (abbreviated injury scale). The usefulness of this concept is enhanced by considering the effective Delta V and PDOF at occupant locations in a vehicle by taking into account the effect of the vehicle rotation. The method of calculating the Delta V at the occupant location is presented in this paper.

Vehicle crush deformation and energy equivalence relationships are widely accepted as technical accident reconstruction tools for estimating the change in velocity (Delta-V) during an impact. Delta-V has been accepted as a basis for evaluating damage severity and potential injury severity. Emori, Campbell and McHenry's work led to CRASH derivative type programs which are based upon a relationship between crush magnitude and Delta-V. SMAC derivative type programs utilize these principles while generating a time dependent analysis (simulation) by maintaining a continuous equalization of forces between the vehicles during the impact phase. This paper reviews basic principles and the relationships between Delta-V, kinetic energy, conservation of momentum, and barrier equivalent velocity which must be adhered to while performing this type of analysis. Several examples and frequently seen misunderstandings of these relationships are discussed.