Multiple factors determine the likelihood, type, and severity of bodily injury following a motor vehicle collision and, in turn, influence the need for treatment, extent of disability, and likelihood of permanent impairment. Among the most important factors is the change in velocity due to an impact (Δv). Other factors include the individual's strength and elasticity, body position at the time of impact, awareness of the impending impact (ie, opportunity to brace, guard, or contract muscles before an impact), and effects of braking. Because Δv is the area under the acceleration vs time curve, it combines force and duration and is a useful way to quantify impact severity. The article includes a table showing the results of a literature review that concluded, “the consensus of human subject research conducted to date is that a single exposure to a rear-end impact with a Δv of 5 mph or less is unlikely to result in injury” in most healthy, restrained occupants. Because velocity incorporates direction as well as speed, a vehicular occupant is less likely to be injured in a rear impact than when struck from the side. Evaluators must consider multiple factors, including the occupant's pre-existing physical and psychosocial status, the mechanism and magnitude of the collision, and a variety of biomechanical variables. Recommendations based solely on patient history and physical findings (and, perhaps, imaging studies) may be ill-informed.