Virtual testing will help us chart a new course in neck injury preventionNovember 28, 2023
How we got here
When we began rating head restraints in 1995, we started with the basics. Neck injuries in rear impacts occur when the head lags behind the accelerating seat and torso. This lag can often be prevented by good head restraint geometry, so our first evaluations were simple measurements using a dummy representing a 50th-percentile man. A restraint should be at least as high as the head’s center of gravity, or about 3½ inches below the top of the head. The backset, or the distance from the back of the head to the restraint, should be as small as possible.In 1995, only 3 percent of the head restraints we evaluated received good geometric ratings, while 82 percent were rated poor. Our ratings led manufacturers to pay attention to these measurements long before a 2010 government standard made good geometry a legal requirement. Good geometry is necessary, but it’s not sufficient. Seats can differ in other ways too, such as structure placement, seatback stiffness and energy-absorbing properties, all of which can affect outcomes for occupants. In 2004 we added a dynamic test for any vehicle with a good or acceptable geometric rating to evaluate how well the seat and head restraint managed crash energy and occupant motion. This test consisted of a simulated rear impact with the vehicle seat mounted to a sled. A special dummy known as BioRID, which has a realistic spine, was buckled in the seat. The pulse used in the test was equivalent to a rear-end crash with a velocity change of 10 mph, or a stationary vehicle being struck at 20 mph by a vehicle of the same weight. The combination of our geometric ratings and our dynamic tests allowed us to identify the most effective head restraints. In a study of real-world crashes, injury rates were 15 percent lower for vehicles with good ratings compared with those rated poor, while long-term injuries, or those lasting three months or more, were 35 percent lower.
Beyond BioRIDSince we began dynamic testing, manufacturers have gotten very good at designing seats for the 10 mph velocity change, and today’s vehicles all perform well in that test. However, there are still differences in real-world performance. Insurance claims data collected by my colleagues at the Highway Loss Data Institute suggest that injury rates in rear-ended vehicles with good head restraint ratings vary widely. So how can we design a new evaluation to better differentiate among restraints? One relatively easy update we intend to make is to add a second dynamic test with a larger velocity change, since many real-world front-to-rear crashes occur at higher speeds. By adding a 15 mph test on top of the 10 mph one, we will be able to glean more information and encourage further progress. We plan to launch a new rating program based on the two tests within the next year or two.
Beyond test speed, other variables are harder to tweak. As is the case with all crash test dummies, BioRID has limited capabilities — for example, it’s only valid for fore-aft motion and lower-severity crashes — and doesn’t represent the diversity of the driving population. While it is an impressive tool with something that closely resembles a human spine, it represents the specific spine of a 50th percentile male.Real-world injury data tell us that women are more likely than men to suffer neck injuries in crashes, but we don’t really know why. Researchers in Sweden are currently developing a female dummy for use in rear-impact testing, which could someday help us evaluate protection for women specifically. However, no matter how sophisticated, physical dummies can’t capture soft tissue and nerve damage, which may play a role in whiplash injuries and in the differences between men’s and women’s susceptibility to them. Perhaps more importantly, any physical dummy, male or female, represents just one particular body type. On the other hand, computer models of the human body, which are currently under development, could be more easily varied to represent a range of body types and injury risk factors. Virtual testing with these models could someday soon provide us with the ability to see how seats and head restraints function for many different people sitting in different positions.