Crash Protection in Rear Impacts: The New Dynamic Tests Using the Hyper-G Sled and BioRID Dummy

By Arthur Croft, DC, MS, MPH, FACO

The comprehensive cost of traffic crashes annually in the United States has recently been estimated to be a stunning $346 billion. That's billions, not millions - more than a quarter of a trillion dollars every year. Of those crashes, rear-impact crashes, while not the most common type of crash, cause the most injuries and by themselves account for $42.9 billion in these comprehensive costs - a cost index that includes economic costs and quality of life losses. The public health burden of these injuries is particularly significant for two key reasons: (a) they affect mostly young people with lots of years of productive life or quality of life to lose; and (b) they are largely preventable, or the effects of these crashes can be significantly mitigated. So whose responsibility is that?

Most readers have heard of the Department of Transportation (DOT). That is your government. One would think they would be working hard to protect us. Under the DOT we have the National Highway Traffic Safety Administration (NHTSA). Largely as a result of the aggressive pioneering work of public safety advocates like Ralph Nader in the late 1960s, a series of Federal Motor Vehicle Safety Standards (FMVSS) was enacted by the newly formed NHTSA to make cars safer for drivers and passengers. Early changes in the form of collapsible steering columns and other countermeasures, which were largely transparent to the motoring public, enjoyed great success in reducing fatalities and serious injuries.

NHTSA has certainly had other successes over the years, and even recently managed to get a new seat and head restraint rule change (FMVSS 202). This new regulation (which was supported by the Spine Research Institute of San Diego during the comments phase) will improve front seat head restraints in passenger cars, pickup trucks, vans, and utility vehicles, and require head restraints in the rear outboard positions, although it won't go into effect for another two years. However, some would argue though that the NHTSA has become an organization with diminishing bite.

Meanwhile, head restraints were mandated as long ago as 1969; why do we need a rule change now? The simple answer is that head restraints have never been adequately designed to effectively prevent whiplash. Moreover, the seat backs haven't had the appropriate physical and mechanical properties to prevent the transmission of serious forces to the necks of occupants. Why? In truth, it really has not been much of a priority for the manufacturers, and the regulations enacted back in 1969 were not detailed enough to guarantee an adequate design. We really didn't know much about whiplash in those days anyway, although that fact does not excuse carmakers from achieving any progress in this area over the past 35 years. Even today, it is well-known that head restraints in most cars offer only marginal whiplash injury protection. Yet even a simple $1 lock can prevent the head restraint from being driven down during a crash, allowing the neck to bend dangerously around the restraint. Why do so many American cars have no such lock? Because $1 times hundreds of thousands of cars is lot of money, and there is no requirement for them to add the lock.

One might ask, if we have known how to make better restraints for so many years, why haven't we made changes in FMVSS 202 by now? For some time, the Europeans have had better legislation in this area. The answer is, among other things, that NHTSA can't compete with these powerful carmaker lobbies. Today, there still is no FMVSS relating specifically to the prevention of whiplash injuries, despite the fact that whiplash is the single largest public health cost in motor vehicle trauma worldwide. There is no cervical spine injury criteria that has been universally adopted, even though the research community has been working on this for some time now.2 Somebody or some organization had to do something about this - and they did.

Beginning in 1995, the Insurance Institute for Highway Safety (IIHS), clearly aware of this ongoing stand-off between lobbyists, manufacturers and the government, decided to take a different approach, called "name and shame." Realizing that one of the most important risk factors vis-à-vis whiplash injury is poor head restraint geometry (i.e., the horizontal distance between the back of the occupant's head and the front of the head restraint, or backset; and the height of the restraint behind the head, or topset), they launched a program in which they measured this geometry using a head restraint measuring device - a type of dummy or phantom placed in the seat. The results were predictably pathetic, with the vast majority of cars receiving "poor" ratings. These were posted on the IIHS Web site. In effect, while no legislation mandated the manufacturers do anything about it, it was hoped that they would be motivated by the bad press coming from this highly regarded, nongovernmental research organization - and it has been effective.

On a yearly basis, head restraints began to improve. There wasn't as much improvement as we would all have hoped for, but some improvement is better than the status quo. The IIHS, by the way, also has a high-speed offset deformable barrier test that compliments NHTSA's own New Car Assessment Program (NCAP), itself a non-obligatory frontal test to beef up the current 30 mph FMVSS 208. This NCAP was itself a compromise strategy since NHTSA couldn't overcome the auto manufacturer's powerful lobby and change the existing 30 mph FMVSS 208 frontal test to a 35 mph test as it wanted. Now, while the test is not mandated, the manufacturers become apoplectic when one of their vehicles doesn't receive the full five-star rating, and they work hard to remedy the situation.

Do these tactics really work? Yes, says Brian O'Neill, president of the IIHS, who spoke recently at CRASH 2004. Over dinner he informed me that they continue to make steady progress over the years with the "name and shame" approach.

NCAP has likewise had its effects, although not all of them were intended and not all are good. For example, it is clear that they have increased the likelihood of whiplash injury in lower speed crashes, because they have forced the auto makers to make their cars stiffer in order to withstand the more challenging crash tests at higher speeds. The 40 mph IIHS test has also contributed to this phenomenon - all the more reason to concentrate on whiplash protection now by improving seats and head restraints.

The IIHS is a member of a larger consortium of insurers and other agencies that are keenly interested in reducing the risk of whiplash in low-speed crashes. This consortium is known as the International Insurance Whiplash Protection Group (IIWPG) and includes Thatcham in the United Kingdom, GDV in Germany, Folksam Insurance in Sweden, the IAG in Australia, ICBC in Canada, and Cesvi in Spain. Most of these are groups structured similar to the IIHS, which is itself a consortium of American insurers.

Many of you may have recently seen Brian O'Neill on Dateline NBC discussing their latest set of tests addressing rear impact safety, but you might not have noticed that there have been some significant changes in the procedures. Recently, I visited with David Zuby, the vice president of IIHS, at their vehicle research center in Virginia. They have a new Hyper-G sled which allows them to recreate a typical crash pulse of a 10 mph delta V rear impact crash. This is a crash in which we would expect whiplash injury in about 50 percent or so of occupants. To this sled is mounted a vehicle seat; in the seat is placed the new BioRID (biofidelic rear impact dummy). [This is one of the dummies we have tested against live human volunteers and found to be the most biofidelic, and it is poised to be adopted by most agencies conducting this type of research.]

The new testing procedure is now a two-step process. First, the static head restraint geometry is measured. If the geometry is either "marginal" or "poor," no further tests are done with the seat and it receives a "poor" overall rating. (If you guessed that the manufacturers are not happy about this, you are most correct.) If, however, the seat and head restraint manage to get an "acceptable" or "good" static rating, the dynamic test using the Hyper-G sled is then conducted.

What do you have to do to pass the dynamic test? There is a 9.5g limit for the thorax and a 70 msec limit on the time it takes the head to contact the restraint, with sooner being better. In order to get the best dynamic rating, head restraint contact should be fast and acceleration should be low. Scoring within the lowest 30th percentile of neck forces earns you a good rating; scoring above the 75th percentile earns you a poor dynamic rating. In order to get a "good" overall rating, both tests have to give a "good" rating.

I asked Matthew Avery, the crash laboratory manager of Thatcham (the Motor Insurance Repair Research Centre) how they settled on these values. He told me that since nobody knows what the thresholds for tolerance really are, they wanted to avoid these polemics so they simply developed these guidelines based on what we do know about whiplash: good head restraint geometry will reduce the likelihood of injury, and the less acceleration, the better. They then simply rank them according to their performance on the basis of percentiles of neck forces rather than on some arbitrary specific values. In the end, I think this is the best way to go for now.

I spent the day with Matthew this past week and observed their set-up as well. Like IIHS, they use the same sled and dummy and post their "name and shame" results on their Web site for all the world to see. In fact, the IIHS and Thatcham Web sites actually share some data, but readers should use caution in interpreting the Thatcham Web site data since European models and cars intended for import to the United States may vary significantly. (To see the results of the current tests, go to or This is an important resource for prospective car buyers.

Most importantly, Matthew tells me, the manufacturers in Europe are acutely interested in these tests. Many car makers, such as Volkswagen (VW), are now requiring the original equipment manufacturers they purchase their seats from to be sure that their seats get high ratings before they (VW) will purchase them. So now the OEMs (original equipment manufacturers) are scrambling for high marks from Thatcham and shouldering the R&D costs themselves, since a seat contract with, say VW, could potentially be worth tens of millions of euros. One manufacturer representative I spoke with this week informed me that this was his third trip to the research facility in several months. As for my trip to IIHS, the car I saw crashed that day was there for its third attempt at a good rating this year. It is obvious that the "name and shame" tactic works.


  1. Zaloshnja E, Miller T, Council F, Persaud B: Comprehensive and human capital crash costs by maximum police-reported injury severity within selected crash types. 48th Annual Proceedings of the Association for the Advancement of Automotive Medicine, Key Biscayne, Fl, Sept 13-15, 2004, 251-264.
  2. Croft AC, Herring P, Freeman MD, Haneline MT: The neck injury criterion (NIC): future considerations. Accid Anal Prev 34(2)247-255, 2002.

Arthur Croft, DC, MS, MPH, FACO
Director, Spine Research Institute of San Diego
San Diego, California

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