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Crash Speeds and Injury Risk: Epidemiologic and Forensic Considerations

Crash-related injuries are the most frequently litigated type of injury in the United States, primarily due to two facts: motor vehicle crashes are one of the most common sources of injuries in society, and many crash-related injuries result from negligence of another individual. Litigation naturally produces a polar alignment of opinion, with one side favoring the plaintiff and the other side favoring the defense. There is no question as to which side has the greatest economic leverage, as auto insurers are multibillion dollar corporations with the added advantage of legislatively-mandated insurance coverage for any vehicle being operated on a public roadway. Thus, if an individual is legally operating a motor vehicle in the United States, that individual almost certainly is paying premium dollars to an insurance company.

               For many years, insurers have invested premium income in a variety of financial instruments and in times of positive economic growth this has proven an effective strategy for increasing their income. In times of stagnant or negative economic growth, insurers must look for other ways to increase profits to satisfy cost-of-living increases for their employees, as well as continuing profits for their owners and investors. This has led to a more recent method, which is to aggressively reduce payments on claims. One of the most effective methods for reducing such payments is to deny payment based on an allegation that a claim is fraudulent or “built up.” No one would argue with the premise that an insurer has the right to protect itself from fraudulent claims, and insurers have capitalized on this right by expanding their definition of what they consider as a fraudulent or built up claim, definitions that are not necessarily validated with data indicating that such claims are necessarily specious.

               A prime example of this expanded definition is the minor impact soft tissue (MIST) claims management protocol initiated by Allstate Insurance in the mid-1900s and adopted by virtually every other US insurer to one extent or another. The premise of this program is that the amount of vehicle damage is an indicator of whether or not a crash could have or should have resulted in an injury. Thus, according to claims adjuster training, an occupant who claims an injury with no more than bumper damage to his or her vehicle is automatically considered to be a likely fraud, despite the presence of medical documentation of injury. The result of this strategy is that, without any valid scientific medical documentation to support their position, the insurance industry has drawn an arbitrary line as to the type of injury claims they will pay versus those they won’t. The real criterion is more likely based in the types of cases insurers believe they are most likely to win in front of a jury: those in which an injury claim can be made to appear counterintuitive or contrary to the “common sense” of a layperson.

               The defense of the litigation of such cases has generated an entire cottage industry in and of itself, with “biomechanists” and “biomechanical engineers” (increasingly referring to individuals with little or no training in these fields) claiming that injuries cannot occur in minimal damage crashes because experimental studies of human volunteer crashes and cadaver, dummy, and animal experiments have somehow proven otherwise. The basis of the MIST defense is the precept that there is a minimum force threshold below which no injury is likely to occur, and that the force threshold coincides with a round money figure in vehicle damage (typically $1000 or less) regardless of the vehicle. Recent trades in litigation practices have illustrated the fact that minimal damage crashes are merely a beachhead in injury threshold defense practices. An increasing practice for many defense experts is to claim that no injury is possible in minimal damage crashes and that no permanent injury is possible in larger crashes without fracture or dislocation.

               Experts who regularly work for the defense in crash injury litigation must rely on an insistence that there is a legitimate scientific basis from which to claim that a crash injury threshold below which injury cannot or is unlikely to occur exists for certain crashes. This premise is fallacious, as numerous epidemiologic studies demonstrate that a variety of injuries can and do occur in a variety of crashes. However, many experts obfuscate this fact with references to government standards and research that refers to injury thresholds. The use of technical jargon to obscure a complete lack of valid basis underlying opinion is called “junk science,” and courts regularly exclude such testimony once it is revealed as such. The purpose of this chapter is to help the reader understand the foundations for references to “injury thresholds,” how they are developed, and why they are not applicable to victims of real-world crash-related trauma.

               While the information in this chapter was derived from a variety of sources, the majority of data regarding real-world crashes have been published by the National Highway Traffic Safety Administration (NHTSA) from the data collection efforts represented in their National Automotive Sampling System (NASS). Additionally, NHTSA collects data on vehicle crashworthiness with their frontal and side barrier tests. The literature cited in the chapter comes from the Society of Automobile Engineers (SAE), the Association for the Advancement of Automotive Medicine (AAAM), the International Research Council on the Biomechanics of Impact (IRCOBI), as well as a variety of trauma-related biomedical journals.

               From the onset, the reader should understand that the purpose of examining injury thresholds is to aid in the design of safer vehicles, and it is to this end that all such criteria have been designed. No injury criterion has ever been developed with the intention of denying the likelihood of injury in an occupant after one has been diagnosed by a clinician, yet this is how it is currently used in the defense of crash injury litigation.

               The majority of crashes occur at relatively lower speed impacts, and as the speed change of collisions increases there are significantly fewer collisions. The majority of injuries also occur in relatively low speed crashes, although the most serious injuries do occur in higher speed crashes. The greatest proportion of the crash injury literature focuses on the minority of crashes that result in more severe to fatal injuries, and thus the literature is somewhat at odds with what the average clinician sees outside the emergency room on a regular basis.

               In the succeeding parts of this chapter that deal with the literature on injury thresholds, it will become readily apparent that these “thresholds” were not designed to be, not can they be used as a means of determining whether a specific  injury may have occurred in a specific case. As mentioned earlier, thresholds are derived from experimental rather than observational data, and are used to design and test vehicles for safety. The term or concept of “threshold” requires explanation anytime it is used. For example, it can refer to the level of frontal crash severity at which it is estimated there is less than an 80% risk of serious head injury, or it can refer to the speed change in a rear impact collision below which an injury to a crash test volunteer is highly likely to sustain injury. Without the specific parameters of the threshold, it is meaningless information.

               This is not to say that the use of experimental data does not allow for the assessment of relative injury risk prospectively for certain crashes. For example, it can be reasonably projected that, in a vehicle with a low head injury criterion (HIC) score in frontal crash test, there will be fewer severe head injuries than in a vehicle with a higher HIC for equivalent crash severities. However, it cannot be concluded that no serious head injuries will occur in the vehicle with the lowest HIC--regardless of collision speed--since the specific conditions of a crash and occupant cannot be predicted with 100% accuracy.

               The most egregious application of injury thresholds is as a means of denying the possibility of a medically-proven injury. Probability or risk of injury is only a tool for projecting a distribution of outcomes, and can never be used retrospectively to deny an outcome. After an injury has occurred in a crash and been medically validated, the “risk” if injury is 100% and it cannot be altered by prior outcomes. The appropriate use of probability can be compared with the game roulette. For a roulette wheel with 100 slots, it is reasonable to conclude that the pre-spin odds of the ball dropping into any particular slot are 1 in 100. After the ball drops into a particular slot the pre-spin odds are irrelevant to the outcome.