Seats and Restraints:
The IIHS doesn’t perform rear crash tests, but instead tests how the seats and headrests can prevent whiplash. This is done by using a special dummy with a realistic spine. That dummy is strapped into the seat of the car. But that seat isn’t in the car as you might expect. Instead, the seat is placed on a sled that is moved in a way that simulates rear-impact with a velocity change of 10 MPH, which is equivalent to a stationary vehicle being struck at 20 MPH.
The engineers first assess the seat geometry, ensuring that it lines up with a dummy that represents the an average male adult. For example, one criterion is that the 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. If the seat lines up to the neck, head and torso correctly, it will be given a Good or Acceptable rating.
Seats rated as good or acceptable, are then put on the sled that simulates the rear-end crash. The engineer’s measure the time needed by the head restraint to react to the velocity change and how well controlled the dummy’s torso is when the “impact” occurs. Additionally, the maximum neck shear force and maximum neck tension are measured on the dummy during the test. These forces help identify how well or poorly an occupant’s head and neck would be supported in a rear impact.
All these factors contribute to the rating for the head restraints and seats. An older study conducted by the IIHS determined that drivers of vehicle with “Good” head restraints were 24 percent less likely to sustain neck injuries in rear-end crashes than drivers in cars with a “Poor” rating.