If you're interested in car safety, then you're probably checking out crash test ratings to find out how well the cars you're considering will hold up in a collision. But how are the ratings computed? Specifically, how does the Insurance Institute for Highway Safety (IIHS) reach its ratings of Good, Acceptable, Marginal and Poor? We have the answers.

IIHS Tests

Like the federal government's National Highway Traffic Safety Administration (NHTSA), the nonprofit IIHS puts cars through several tests. Unlike NHTSA, IIHS doesn't offer an overall score. In other words, while NHTSA might give a car five stars overall, IIHS prefers to display a car's rating in each of its six tests, with no overall average given. With that said, top-performing cars can earn a top-pick rating, something we've covered below.

All IIHS tests use the same measuring system. The best cars earn a Good rating, while the worst earn Poor. In between, IIHS rates cars as Acceptable or Marginal, with Acceptable being better. IIHS ratings are not weighted based on other cars in the class but rather assigned based on performance and likelihood of injury. That means there's no bell curve; if every car in a class does poorly on a test, IIHS results will reflect that.

Side Test

IIHS's side-impact assessment is one of its five tests. In a side test, cars are struck from the side at 31 miles per hour by a 3,300-lb barrier with SUV bumper height. There are two crash-test dummies: an adult in the driver's seat and a child in back. To provide a rating, IIHS engineers examine driver and passenger injury measures, head protection and the vehicle's structural integrity.

Roof-Strength Test

In the roof-strength test, a metal plate is pushed against a car's roof at a slow, constant speed. There aren't any dummies in this test, and the test's sole purpose is to determine how well a car's roof would hold up in a rollover accident. For a Good rating, a car must be able to withstand a force of at least four times its own weight before the roof is crushed by 5 inches.

Small- and Moderate-Overlap Front Tests

In the minds of most drivers, a crash test involves a car crashing head-on into a fixed barrier. IIHS performs that test but with a slight variation: The barrier isn't hit by the entire front of the car. Instead, only a portion of the car hits the barrier, and that portion varies in each of these two tests.

In the moderate-overlap test, 40 percent of a car hits the barrier. In the small-overlap test, 25 percent of the car hits the barrier. The small-overlap test has been especially challenging for automakers, largely because it involves such a small portion of the car colliding with the barrier. That means the car can't use its full crash structure to absorb the impact, which has led to many Poor scores.

Head-Restraints and Seats Test

IIHS's last test examines a car's head restraints and seats. This one doesn't involve a car smashing into any barriers. Instead, it tests a car's likelihood to give you whiplash and neck sprains in an accident. The test examines where a car's head restraint is located relative to the head of an average adult male; vehicles that score well have the restraint around 3.5 inches below the top of the head.

Top Safety Pick and Top Safety Pick+

While IIHS doesn't provide overall ratings or averages of its crash test ratings, the nonprofit group does single out cars that do especially well. These cars are given one of two designations: Top Safety Pick and the slightly more prestigious Top Safety Pick+.

To earn a Top Safety Pick rating, cars must be rated Good in the organization's side, roof-strength, head-restraints and moderate-overlap front tests, as well as Good or Acceptable in the new small-overlap front test. A Top Safety Pick+ rating is awarded to cars that offer a strong rating in a separate front-crash-prevention assessment. That assessment simply considers whether a car has standard or optional features that will prevent or warn drivers about an oncoming collision.

author photo

Doug DeMuro has a wide range of automotive industry experience, from work at a Ferrari dealership to a manager for Porsche North America. A lifelong car enthusiast, Doug's eclectic vehicle purchases include a Porsche 911 Turbo, an E63 AMG wagon, an old Range Rover and a Mercedes Benz G-wagen.

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