The federal agency’s side-impact test follows a similar procedure, though with dummies belted into front and rear seating positions. Instead of the vehicle moving, a 3,015-pound barrier is bashed into the side of the vehicle at 38.5 mph to simulate a “T-bone” intersection collision. The NHTSA measures the forces involved but does not consider the likelihood of a head injury.

The NHTSA also conducts a rollover resistance test, which measures how easily the vehicle might roll over in a crash that does not involve another car — for instance, the likelihood that a vehicle might be “tripped” by a curb, guardrail or ditch. Here, the agency relies on two factors in the test: a calculation from measurements at rest, called the static stability factor (SSF), and a dynamic “tip-up” test, which is a sudden driving maneuver on a track under controlled conditions.

In the IIHS’ frontal offset crash test, a dummy is mounted into the driver’s seat and 40 percent of the total width of the vehicle strikes a barrier on the driver’s side at 40 mph. The barrier is made of deformable aluminum honeycomb, so as to simulate the crash against another vehicle. Tested vehicles are then ranked based on structural performance and the likelihood of occupant injury. The test primarily measures how well the front-end crush zone absorbs energy during a crash and how well the occupant compartment holds together.

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The IIHS side-impact test is similar to the NHTSA test, though the impact occurs higher than in the federal test to more accurately simulate a collision from a pickup or SUV. The barrier weighs 3,300 pounds and is impacted at 31 mph. Test dummies used in the side-impact test are the size of a small fifth-percentile female or 12-year-old adolescent to measure the worst-case scenario head injury from the front of the oncoming vehicle. Again, injury measure, intrusion of the vehicle and the motion of the dummies are studied to arrive at ratings.

The IIHS does not test for rollover likelihood, but the Institute is now testing for the likelihood of head or whiplash injury as the result of a rear-end collision — a particular concern for the insurance industry. Here, the organization looks at the geometry of the seat’s head restraint, and the seats are also tested in a situation that simulates a stationary vehicle being struck at 20 mph by a vehicle of the same weight. Forces are measured through a special test dummy, and the geometric and dynamic ratings are combined.

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