IIHS research bibliography

The Insurance Institute for Highway Safety rates vehicle seat/head restraint designs as good, acceptable, marginal, or poor using a protocol by the Research Council for Automobile Repairs’ International Insurance Whiplash Prevention Group (RCAR/ IIWPG). Studies of insurance neck injury claim rates for rear impact crashes show that vehicles with seats rated good have lower claim rates than vehicles with seats rated poor, but the relationship between acceptable/ marginal ratings and claim rates is less clear. To better understand the relationship between measured neck injury criteria and injury claim rates, a series of rear impact crash tests was conducted to determine the influence of crash pulse, as dictated by vehicle structure, on the performance of seat/head restraints. The role of head restraint adjustment also was examined by comparing BioRID responses in the driver position, with the restraint adjusted according to the RCAR/ IIWPG protocol, and in the front passenger position, with the restraint adjusted to its lowest position. In an attempt to match the severity of the RCAR/IIWPG crash pulse, vehicles were struck by a flat rigid barrier to create a velocity change of 16 km/h (10 mi/h). Four small cars with rated seat/head restraints and varying real-world neck injury claim rates were selected. The 2006 Honda Civic and 2005 Chevrolet Cobalt both received good ratings in the RCAR/ IIWPG sled test, but the Civic had a relatively low neck injury claim rate compared with the Cobalt. The 2006 Saturn Ion and 2005 Ford Focus both received marginal ratings in the sled test, but the neck injury claim rate for the Ion was comparable with that for the good-rated Civic, and the Focus had the highest neck injury claim rate among the vehicles tested. BioRID response ratings for the driver position matched the sled test ratings for the Cobalt and Focus but were one rating level lower for the Civic and Ion. BioRID response ratings for the passenger position were the same as those for the driver position for all vehicles except the Cobalt, which was one rating level lower. The findings suggest that changing the RCAR/ IIWPG protocol to include vehicle specific crash pulses and/or changing the restraint setup would not improve the relationship between seat/head restraint ratings and neck injury claim rates. Furthermore, examination of additional BioRID injury metrics not currently assessed under the protocol does not help explain real-world neck injury claim rates and does not support changing the current evaluation criteria. Additional research is needed to determine whether vehicle underride/override alters vehicle accelerations in a way that makes crash tests more predictive of neck injury claim risk in rear-end collisions.

A series of simulated rear crashes conducted according to the seat/head restraint evaluation protocol of the Research Council for Automobile Repairs International Insurance Whiplash Prevention Group (RCAR/IIWPG) examined the effects on BioRID responses of changes in seatback configuration and rotation about the recliner pivot. In general, a more compliant seatback design that allowed the torso to sink-in as the seatback frame was accelerated forward was beneficial toward reducing forces measured in the dummy’s neck. The design of comfort features like lumbar supports influenced this seatback characteristic. Seatbacks that rotated less produced lower neck forces but higher T1 acceleration and NIC, suggesting a possible design conflict that might be resolved by attention to other seat design characteristics like seatback stiffness or initial backset. Comparison with earlier research by Locke et al. (2009) using the same set-up and seat back designs indicates the feasibility of seat/head restraint designs that satisfy the conditions for a good RCAR/IIWPG rating and the requirements of the Federal Motor Vehicle Safety Standard 202 dynamic compliance option.

This paper offers a historical review of vehicle design measures that have been implemented to reduce neck injury risk to occupants of rear-struck vehicles. In particular, the rationale behind ratings published by the Insurance Institute for Highway Safety (IIHS) is described. The paper also describes how seat and head restraint designs in US vehicles have changed during the 6 years IIHS has been rating passenger vehicle seats. Efforts to understand how well these ratings predict real crash injury risk are reviewed.

Objective: This article offers a historical review of vehicle design measures that have been implemented to reduce the risk of neck injuries to the occupants of rear struck vehicles.Methods: The literature on regulations, consumer information programs, and efforts by vehicle manufacturers to address whiplash injuries is summarized along with studies evaluating the efficacy of the resulting vehicle design changes.Results: Vehicle designs and, in particular, the designs of seats and head restraints have changed considerably over the last 40 years. With varying degrees, these changes seem to be reducing the likelihood that occupants in rear struck vehicles will suffer neck injuries in such crashes.Conclusions: Vehicle design has influenced the risk of neck injuries in crashes, and future design changes offer potential for further risk reduction.

The SAE J826 H-point machine was designed to measure occupant accommodation dimensions relative to a loaded seat. It has become an intrinsic part of various crash dummy set up processes, but it has never had a formal calibration procedure. Whilst H-point location appears to be consistent from one device to another, the weight hanger locations show greater variability, and this can consequently affect the height and backset measurements of head restraints taken with a head restraint measuring device mounted upon the weight hangers. This paper describes the development of a calibration procedure and jig to measure the location of the weight hangers so that adjustments can be made if necessary. This procedure and calibration tool will enable more consistent seat evaluations, dummy set up, and consistently effective anti-whiplash seat designs.

Objectives: The Insurance Institute for Highway Safety assigns consumer safety ratings to passenger vehicle seats based on laboratory sled tests that simulate rear-end collisions. The purpose of this research was to determine how well these ratings correlate to driver neck injury risk in real-world crashes.Methods: Insurance claims for cars and SUVs struck in the rear by the front of another passenger vehicle were examined for evidence of driver neck injury. Logistic regression was used to compare neck injury rates for vehicles with different seat ratings while controlling for other important variables.Results: Driver neck injury rates were 15% lower for vehicles with seats rated good compared with vehicles with seats rated poor. Rates of driver neck injuries lasting 3 months or more were 35% lower for vehicles with seats rated good compared with vehicles with seats rated poor.Conclusions: Seat/head restraints that perform better in dynamic sled tests have lower risk of neck injury than seats that rate poor, especially when considering long-term injuries. However, the relationship of dynamic seat ratings to neck injury rates is not linear. Further research is needed to determine whether the criteria for rating seats can be amended so as to be more uniformly predictive of real-world neck injury.

Since 1969 motor vehicles in the United States have been required to have head restraints in front seats to mitigate neck injuries resulting from rear crashes. Simply equipping cars with head restraints reduced the incidence of these injuries by as much as 18 percent (Kahane, 1982; O’Neill et al., 1972; States and Balcerak, 1973). Further injury reductions were realized as head restraint designs evolved to be taller and closer to the backs of occupants’ heads in response to seat ratings published by the Insurance Institute for Highway Safety (IIHS) and other members of the Research Council for Automobile Repairs (RCAR) (Chapline et al., 2000; Farmer et al., 1999; Farmer et al., 2003). Led by Saab and Volvo in the 1990s, vehicle manufacturers began fitting more advanced seat designs specifically to address whiplash in rear crashes. These also were shown to be effective at reducing neck injury risk (Farmer et al., 2003; Jakobsson and Norin, 2004; Viano and Olson, 2001). Despite these improvements whiplash, or minor injury to the neck, is one of the most common consequences of motor vehicle crashes, affecting nearly 1 million people annually in the United States. Many of these injuries still occur in rear crashes (National Highway Traffic Safety Administration, 2004).

Since 1995 the Insurance Institute for Highway Safety (IIHS) has measured and evaluated the static geometry of head restraints on vehicle seats. Geometry is important because a restraint positioned behind and close to the back of an occupant’s head is a necessary first step toward reducing neck injury risk in rear crashes. In recent years head restraint geometry in new model passenger vehicles has improved steadily. However, a restraint that does not remain close to the head during a crash cannot effectively support the head and neck, so the effectiveness of a restraint with good static geometry may be reduced by poor dynamic response of a seatback or restraint cushion. In addition, the effectiveness of advanced seat and head restraints designed to move during a crash, either to improve geometry or reduce torso accelerations, can be evaluated only in dynamic tests. Thus, good geometry is necessary but, by itself, not sufficient for optimum protection. Dynamic evaluations using a test dummy also are needed to assess protection against neck injury in rear crashes. Several insurance-sponsored organizations formed the International Insurance Whiplash Prevention Group to develop a seat/head restraint evaluation protocol, including a dynamic test. Tests using this protocol produce substantially different results among seat/head restraint combinations, even among those with active head restraints. IIHS published its first set of evaluations using the protocol in fall 2004. This paper describes the rationale behind the protocol and summarizes the results of IIHS testing so far.

Automobile insurance claims were examined to determine the rates of neck injuries in rear-end crashes for vehicles with and without redesigned head restraints, redesigned seats, or both. Results indicate that the improved geometric fit of head restraints observed in many newer vehicle models are reducing the risk of whiplash injury substantially among female drivers (about 37% in the Ford Taurus and Mercury Sable), but have very little effect among male drivers. New seat designs, such as active head restraints that move upward and closer to drivers' heads during a rear impact, give added benefit, producing about a 43% reduction in whiplash injury claims (55% reduction among female drivers). Estimated effects of Volvo's Whiplash Injury Prevention System and Toyota's Whiplash Injury Lessening design were based on smaller samples and were not statistically significant.

The magnitude of crash acceleration affects neck injury risk in rear crashes. It is less clear whether different shaped crash pulses of similar overall severity also cause different injury outcomes. This study examined two crash pulse shape characteristics and their effects on dummy responses in rear crashes: 1) oscillations of approximately twice the frequency of the main mode typical of full-scale crash tests and 2) timing of the maximum peak acceleration. The presence of acceleration oscillations did not influence the magnitudes of BioRID responses. The timing of peak sled acceleration did affect both the timing and magnitude of some BioRID responses.

Comparisons of whiplash injury risk among different vehicle seat/head restraint designs using dynamic tests requires consideration of rear-crash-pulse characteristics in a range of low to moderate crash severities. Insurance damageability tests show that for many European cars the rear structures are becoming stiffer and tend to produce crash pulses with higher peak accelerations occurring earlier than was observed with older designs. In addition, many modern vehicles have a distinctly bimodal rear-end crash pulse. The influence of these pulse-shape characteristics on neck injury risk is unknown. In this study, 21 sled tests were conducted with new current-model car seats supplied by three different vehicle manufacturers. Each seat was subjected to one of seven different rear crash pulses that varied in maximum acceleration (7.1-9.5 g), delta V (16-20 km/h), and shape (unimodal versus bimodal with different acceleration rates). One of the bimodal pulses was an average of pulses recorded in a number of approximately 16 km/h delta V car-to-car crash tests. One of the unimodal pulses approximates the trapezoidal shape proposed by the International Insurance Whiplash Prevention Group. BioRID neck responses measured in these tests included NIC, Nkm, Nij, and NDC. Analyses show that higher accelerations produced consistently higher NIC, Nij, and Nkm responses, but the other injury measures did not exhibit the same pattern. Nij and Nkm also increased with delta V, but other injury measures did not. The effects of pulse shape for all injury measures were unclear. Although all of the injury measures varied as the test-pulse shape was changed, only some injury measures exhibited consistency across the different seat models tested, so it is not possible to know whether the variation in injury measures was due to test pulse differences or only reflected underlying test-to-test variability.

This paper describes a test to assess the postural comparability between the anthropometric dummy BioRID II, the head restraint measuring device HRMD, and humans of similar size. Volunteers were monitored while driving vehicles along a course which involved certain movements. Measurements were made of each volunteer's head relative to the head restraint. The driver's seat position and angles were also measured. It was found that seat back angles were more upright than those used for a dummy but the difference was not great. Seat adjustment meant that the dummy tended to sit lower in the seat than the human, making a difference to the position of the head restraint relative to the head. The study concluded that the seating positions of BioRID and HRMD were appropriate.

In a rear-end crash, if an occupant's head is unsupported it lags behind as the torso is accelerated forward. This causes the neck to change shape, first taking an s-shape and then bending backward in a 'whiplash' motion. This sudden differential movement of the head and torso can cause 'whiplash' injuries to the neck. This paper reviews methods to minimize the differential head/torso movement and reduce the resulting injuries, focusing on the necessary first step for prevention, which is a head restraint that is behind and close to the back of an occupant's head during the crash. The history of head restraints since the 1950s is reviewed, with particular attention to advanced restraint designs that are proving effective in reducing whiplash injury risk in dynamic tests using a new crash test dummy neck and a new neck injury criterion.

This two-year investigation was designed to estimate the incidence of driver neck pain in rear-struck vehicles involved in two-vehicle collisions and to determine the relationship between neck pain and specific vehicle, human, and environmental factors. Neck pain percentages were significantly higher for female (45%) than for male (28%) drivers. For female and male drivers, neck pain likelihood increased as head restraint height decreased below the head's center of gravity, although this effect was significant only for females. Head restraint backset, the horizontal distance measured from the back of the driver's head to the front of the head restraint, was not found to be related to neck pain for female drivers. Backset trends for male drivers could not be evaluated because few male drivers had head restraints that were high enough for backset to be relevant. Reported neck pain decreased for older drivers (females only), drivers in less severe crashes, and drivers in heavier cars (females only); all head restraint analyses were adjusted for these characteristics. Women, and most likely men, in the United States would benefit greatly from international harmonization to European head restraint standards. Until then, both women and men should be encouraged to adjust their adjustable head restraints, if possible, behind their heads' centers of gravity and to sit with the backs of their heads as close as possible to their head restraints.

Insurance claims were examined for evidence of neck injuries to drivers of passenger cars struck in the rear. Neck injury rates were significantly lower for male drivers, elderly drivers, and drivers in less severe crashes. Even after accounting for differences in driver demographics and crash severity, neck injury rates were significantly lower for drivers of cars with head restraints that were more likely to be behind the heads of motorists.

A BioRID (biofidelic rear impact dummy) representing a 50th percentile adult male was seated in the front passenger seat of six new vehicle models in a series of low-speed crash tests. The neck injury criterion (NIC) and other dummy responses that may indicate whiplash injury risk were recorded. Both front-into- rear and rear-into-barrier tests with an average velocity change of 11 km/h were conducted. Head restraints were tested in both adjusted (up) and unadjusted (down) positions. Damage to all models was minor, and longitudinal vehicle accelerations were low (less than 7 g). Neck extension angles and bending moments were much less than injury assessment reference values (IARV) (80 degrees and 57 Nm, respectively), indicating low risk of hyperextension injuries. Neck tension and transverse forces also were less than IARVs used to indicate the risk of more serious neck injuries. NICs recorded on the BioRID during these tests were in the range recorded from volunteers who did not experience long-term injury, and most were below the proposed IARV of 15 m^2/s^2. NIC and neck tension were lowest in tests of the Saab 9-3, which was equipped with an active head restraint designed to move upward and forward in a rear-end crash. In general, adjusted head restraints positioned to have a small horizontal gap between the restraint and dummy''s head tended to produce lower injury measures. However, none of the recorded injury measures exhibited any influence of head restraint height. Both NIC and neck tension seemed to be influenced by crash severity indicated by both vehicle and pelvis acceleration.

Head restraints, intended to reduce the risk of neck injury in rear impacts, have been required by Federal Motor Vehicle Safety Standard 202 in new passenger cars since the 1969 model year and in light trucks since the 1992 model year. This standard specifies minimum geometric criteria for head restraint height and width, but actual head restraints vary greatly in terms of height, adjustability, and proximity to the backs of people’s heads. To reduce the likelihood of whiplash injury, a head restraint should be behind and close to the back of an occupant’s head to provide support in a rear impact. Head restraints in 247 different 1997 model year passenger vehicles were measured to evaluate their likely proximity to the head of an average-size male occupant. Seven vehicles (3 percent) were rated as having good head restraint geometry, whereas 156 vehicles (62 percent) only offer seats with poor head restraints. Thirty-eight percent of the vehicles offer seats with marginal or better head restraints. Results of this evaluation highlight the inadequacy of the present standard’s geometric criteria and the need for improved head restraint designs.

Head restraints, which are intended to reduce the risk of neck injury in rear impacts, have been required by Federal Motor Vehicle Safety Standard 202 in new passenger cars since the 1969 model year and in light trucks since the 1992 model year. This standard specifies minimum geometric criteria for head restraint height and width only. To reduce the likelihood of whiplash injury, a head restraint should be behind and close to the back of an occupant's head to provide support in a rear impact. The head restraints in model year 1995 passenger cars and passenger vans,~ere measured to evaluate their proximity to the head of an occupant. Of 164 vehicles measured, only five vehicles (3 percent) were rated as having good head restraints. One hundred and seventeen (71 percent) of the vehicles had poor . restraints, eight were acceptable, and 34 were marginal. The evaluation considered head restraint geometry relative to the head of an average size male, the restraint's adjustability, and it's ability to lock in place. The results of this evaluation highlight the inadequacy of the present standard's geometric criteria and the need for improved head restraint designs.

Properties of the human neck which may influence a person’s susceptibility to “whiplash” injury during lateral impact have been studied in 96 normal subjects. Subjects were chosen on the basis of age, sex, and stature, and data were grouped into 6 primary categories based on sex (F,M) and age (18-24, 35-44, 62-74). Stature served as a secondary variable, with each group of 16 subjects being matched to obtain an average stature close to the 50th percentile for the category. The data include: measures of head, neck and body anthropometry in standing and normal seated positions; stretch reflex time of sternomastoid muscles; head/neck response to low-level acceleration; voluntary isometric muscle force in the lateral direction; and three-dimensional range of motion of the head and neck. Data are presented in a format applicable for biomechanical modeling of the seated human occupant and have been used in the MVMA-2D model adjusted for side impact at 10 and 30 mph to determine the influence of the measured properties on reducing “whiplash” injury susceptibility.

Properties of the human neck which may influence a person's susceptibility to “whiplash” injury during lateral impact have been studied in 96 normal subjects. Subjects were chosen on the basis of age, sex, and stature and data were grouped into six primary categories based on sex (F, M) and age (18-24, 35-44, 62-74). The data include: measures of head, neck and body anthropometry in standing and simulated automotive seating positions, three-dimensional range of motion of the head and neck, head/neck response to low-level acceleration, and both stretch reflex time and voluntary isometric muscle force in the lateral direction. Reflex times are found to vary from about 30 to 70 ms with young and middle aged persons having faster times than older persons, and females having faster times than males. Muscle strength decreases with age and males are, on the average, stronger than females. Range of motion was determined by using three-dimensional photogrammetry to compute Euler angles relative to the Frankfort plane. Ranges of motion are reported for 6 planar movements (flexion, extension, left and right rotation, left and right lateral bend) and 3 combination movements (left rotation plus flexion, left rotation plus left lateral bend, and right rotation plus extension) chosen to simulate typical lateral impact conditions. For the young subjects, ranges of motion for males and females are similar. In all cases the range of motion decreases with age, with the rate of decrease for males being greater than that for females. Data are presented in a format applicable to biomechanical modeling of the seated human occupant.

Analytical man motion models have been used to study how basic physical measurements may relate to susceptibility to cervical hyperextension-hyperflexion injury in an automobile collision. The parameters considered in the computer study are head-neck mass and moments of inertia, anthropometry, neck muscle strength, and location, as well as strength of motion-limiting “stops.” In addition, related environmental parameters such as seat structural properties and crash acceleration pulse have been included. The data used with the computer program span the range of physical and sexual variation in function and structure of the neck in a representative U.S. population and have been obtained in an extensive experimental program. Results are presented which attempt to relate injury susceptibility to physical stature, age and sex.

The cervical hyperextension -- hyperflexion syndrome, commonly referred to as "whiplash", has been well-documented in the clinical and experimental literature. However, it has become so prolific, that review and retrieval have become major research tasks. With the scientific, medical and technical literature expanding at an exponential growth rate, the deluged scientific researcher or physician cannot even begin to adequately keep abreast of, let alone review, the literature. Time limitations created by this problem force the researcher into reviewing only the popular, well known journals. Multidiscipline system approaches are virtually impossible under such conditions, and misconceptions caused by information "inbreeding" are often the end result. Medical doctors not specifically trained in complex engineering principles and engineers not specifically trained in complex medical principles create misconceptions which, when inbred: with even less informed colleagues, produce aberrant mutations of fact. The whiplash literature of today still contains reference to several of these misconceptions generated in the 1950s. Realizing the inherent danger of such an approach, this literature search was undertaken to provide a broad base of information for use of diverse disciplines such as engineer, physician, and anthropologist. The objective of this report, therefore, is to bring together for the first time in a single compilation an up-to-date and comprehensive multidisciplinary literature search related to the cervical hyperextension-hyperflexion syndrome. This comprehensive bibliography produced 2,326 references in surveying 'four major areas of the cervical hyperextension-hyperflexion syndrome.

Basic physical characteristics of the neck have been defined which have application to the design of biomechanical models, anthropometric dummies, and occupant crash protection devices. The study was performed using a group of 180 volunteers chosen on the basis of sex, age (18-74 years), and stature. Measurements from each subject included anthropometry, cervical range-of-motion (observed with both x-rays and photographs), the dynamic response of the cervical flexor and extensor muscles to a controlled jerk, and the maximum voluntary strength of the cervical muscles. Data are presented in tabular and graphic form for total range-of-motion, cervical muscle reflex time, decelerations of the head, muscle activation time, and cervical muscle strength.

The study sample consisted of 67,143 insurance claims; the manual search of the files associated with these claims led to the identification of 6,833 struck cars meeting the study criteria. For these claims the following items of data were recorded for each forward car: model year, manufacturer, sex of the driver, and whether any type of neck or back injury was claimed. It was determined that there was an 18 percent reduction in the frequency of claimed neck injuries to drivers (both sexes combined) in cars with head restraints as standard equipment when compared with the frequency for drivers in cars without head restraints as standard equipment. Similar results by sex of driver are also presented. An analysis by manufacturer revealed differences similar to the overall pattern, but the numbers of observations when divided into these various subgroups were generally too small to support firm conclusions. The results of a visual survey of drivers in 4,983 moving domestic passenger cars with adjustable head restraints in the Los Angeles and Washington, D.C. Metropolitan areas indicated that in the Los Angeles area, 74 percent of the male drivers and 57 percent of the female drivers had the head restraints improperly positioned while in the Washington, D.C. Area 93 percent of the males and 80 percent of the females had the restraints improperly positioned. The study is concluded with a discussion of previous research on whiplash injuries and head restraints.

A parameter study is performed involving several analytical vehicle occupant models in current use, with investigation of neck representations a primary goal. Side, oblique, and rear impact situations are investigated. Attention is given to the effects of varying head-neck mass and moments of inertia, anthropometry, muscle strength, and location, as well as well as strength, of motion-limiting “stops.” A model that replaces the conventional simple ball-joint neck with a two-joint, extensible neck is studied. This model also makes use of joint-stop ellipses to approximate the anatomical range for relatively free angular motion at a joint. Allowance is made for the effect of muscle contraction on occupant dynamics as a function of the degree of voluntary or involuntary tightening of the muscles, based upon experimental findings. A discrete parameter neck model that treats the cervical spine as a linkage of rigid vertebrae and massless, deformable discs is discussed briefly. It is determined that, besides being extensible and having at least two joints, three-dimensional neck representations should account for coupling between the forces resisting the three possible rotational motions-yaw, pitch, and roll-that can occur between the head and the torso.