Neck sprains and strains are the most frequently reported injuries in U.S. auto insurance claims. Such whiplash injuries can be sustained in any type of crash but occur most often in rear-end collisions. Good head restraints can help prevent them.

IIHS tests vehicle seats and head restraints with a special dummy that has a realistic spine. The vehicle seat — with the dummy in it — is placed on a sled, which is moved to simulate a rear impact.

Head restraint geometry

Good geometry is essential for an effective head restraint. If a head restraint isn't behind and close to the back of an occupant's head, it can't prevent whiplash in a rear-end collision. IIHS evaluates the geometry of head restraints in passenger vehicles based on the height and backset relative to an average-size male. A restraint should be at least as high as the head's center of gravity, or about 3.5 inches below the top of the head. The backset, or distance behind the head, should be as small as possible. Backsets of more than about 4 inches have been associated with increased symptoms of neck injury in crashes. The restraints are measured with the angle of the torso at about 25 degrees, a typical seatback angle. IIHS classifies each restraint according to its height and backset into one of four geometric zones — good, acceptable, marginal or poor.

Head restraints have improved since the Institute first began rating them. In 1995, only 3 percent of measured head restraints received good geometric ratings, while 82 percent were rated poor. Among recent models, most head restraints have good geometric ratings, and the rest are acceptable. IIHS ratings have forced manufacturers to pay attention to head restraint design. More recently, a government standard in effect since September 2010 requires a minimum of 29.5 inches from an occupant's hip to the top of a head restraint and a backset of 2.2. inches or less. This guarantees that nearly all new head restraints would be rated good for geometry by IIHS.

Dynamic ratings derived from
seat parameter and neck force results

Seat parameters+Neck forces=Dynamic ratings
Pass+Low=
G
Moderate
A
High
M
Fail+Low=
A
Moderate
M
High
P

Overall ratings derived from
both geometric and dynamic ratings

Geometric rating+Dynamic rating=Overall rating
G
+
G
=
G
A
A
M
M
P
P
A
+
G
=
A
A
A
M
M
>
P
P
M
+No dynamic test=
P
P
+No dynamic test=
P

Dynamic ratings

Seats and head restraints with geometry rated good or acceptable are tested in a simulated rear impact conducted on a sled. The test assesses how well the seats support the torso, neck and head of a BioRID dummy. The test simulates a rear-end crash with a velocity change of 10 mph, approximately equivalent to a stationary vehicle being struck at 20 mph by a vehicle of the same weight.

A seat/head restraint's dynamic rating depends on performance in the sled test. There are two sets of criteria for evaluating performance. The first criteria are the two seat design parameters, time to head restraint contact (must be ≤70 milliseconds to pass) and torso acceleration (must be ≤9.5 g to pass). The second set of evaluation criteria is comprised of the maximum neck shear force and maximum neck tension measured on the dummy during the test. These neck forces (classified as low, moderate or high) indicate how well or how poorly an occupant's head and neck would be supported in a rear impact at low to moderate speed. A seat that passes at least one of the seat design parameters and has low neck forces earns a dynamic rating of good.

Overall ratings

The geometric rating and the dynamic rating are combined to produce a seat/head restraint combination's overall evaluation. A good rating can only be earned with both a good geometric rating and a good dynamic rating. Seats with only acceptable geometry earn an acceptable overall rating even if their ratings in the dynamic test are good. Seats rated marginal or poor for geometry would not be tested dynamically and automatically would be assigned a rating of poor; however, since the 2011 model year, all seats have had good or acceptable geometry.

The IIHS rating applies only to the specific seat/head restraint combination tested, though many vehicles have multiple options for these components. For a given vehicle, the Institute typically tests the seat option most likely to be found on dealer lots.

BioRID

BioRID 50th percentile male rear-impact dummy


Dummy and sled used in dynamic tests

Dynamic testing of seats/head restraints requires a dummy with a realistic spine and neck. Before the development of BioRID, or biofidelic rear impact dummy, all dummies had rigid spines and necks that didn't interact with vehicle seats the way human spines and necks do. BioRID was developed for rear testing by a consortium of Chalmers University, restraint maker Autoliv, Saab and Volvo. This dummy, representing an average-size man, has a spine composed of 24 articulated vertebra-like pieces. The spine interacts with vehicle seats during tests in much the same way as a human spine would. Plus BioRID's segmented neck can produce the motion observed by human necks in real-world crashes in which vehicles are struck from behind.

The device on which dynamic tests of seats/head restraints are conducted is a steel flatbed sled that runs on fixed rails. The sled is moved to simulate vehicle crash accelerations, re-creating the forces on occupants inside vehicles during real-world crashes. The changing acceleration or deceleration over the time duration of a crash is referred to as a crash pulse, and the key aspect of a sled is that it can be programmed to produce specific crash pulses. To evaluate rear crash protection, vehicle seats are affixed to the sled, which is accelerated to simulate a stationary vehicle that's rear-ended by another vehicle of the same weight going 20 mph. To accomplish this, compressed air is pumped into a special cylinder, thrusting a ram forward in a preprogrammed pattern of acceleration (crash pulse). Peak acceleration in the sled test is 10 g (5 g mean acceleration), and the duration is 91 milliseconds.

Understanding the ratings:
How much better are vehicles with good head restraints?

Vehicles with good rear crash ratings do a better job of preventing neck injuries in the real world. In 1999, IIHS researchers analyzed more than 5,000 insurance claims and determined that drivers with head restraints with good geometric ratings were 24 percent less likely than drivers with poor-rated head restraints to sustain neck injuries in rear-end crashes.

A 2008 study looked at real-world crashes involving seat/head restraint combinations that had been dynamically tested. Insurance claims for drivers of cars and SUVs struck from behind were examined for evidence of driver neck injury. Injury rates were 15 percent lower for vehicles with seats/head restraints rated good compared with vehicles with seats/head restraints rated poor. Long-term injuries, or those lasting three months or more, were 35 percent lower for vehicles with seats/restraints rated good compared with seats/restraints rated poor.