NHTSA crash tests

As part of its New Car Assessment Program, NHTSA scores its tests using a scale of one to five stars; the fewer the stars, the greater the likelihood of injury or death. The IIHS uses a four-level scale: Poor, Marginal, Acceptable, and Good.

Front crash

NHTSA's front-crash test accelerates a car straight into a rigid barrier at 35 mph, with the entire width of a vehicle's front end hitting the barrier. Instrument-bearing, seat-belted crash-test dummies in the two front seats record the level of crash forces on the head, chest, pelvis, legs, and feet. Those measurements correlate with injury, but just the head and chest results form the basis of the star rating. Individual star ratings are assigned to the driver and the front passenger.

Some automotive experts have criticized NHTSA's full-frontal, rigid-barrier test as unrealistic because such head-on crashes into a solid flat object are rare. Others argue that real world or not, flat-barrier testing is a good way to gauge the effectiveness of the restraint systems. These days most vehicles get four or five stars in this test.

Side crash

NHTSA's side-impact test simulates a vehicle struck on the left side by a 3,015-pound car traveling at 38.5 mph. Such a scenario mimics what could happen if you are hit on the side when you're easing into an intersection. The side-impact star-rating scores, for the driver and left-rear passenger, indicate the chance of a life-threatening chest injury.

NHTSA actually records several other injury points but at this time it doesn't use them to derive the star rating. The one exception is a serious head injury. If the force recorded at the dummy's head suggests a potentially fatal injury, NHTSA notes that as a "safety concern" in its official publications.