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What Do Crash Safety Ratings Really Mean?

It's never a pleasant thought, but the reality is that car accidents are sometimes unavoidable. You can ease some of your on-road anxiety by knowing (ahead of time) how your car's safety features will perform in the event of a collision.

No one ever buys a car thinking they’re going to wreck it. Preventing a crash and protecting occupants should one occur is of top concern for most car shoppers. That's where crash safety ratings come in. Understanding what they mean and how they are calculated can help you evaluate your options more effectively and give you more confidence in the shopping process.

Who Rates the Cars?
The National Highway Traffic Safety Administration (NHTSA)1, a federal government agency, and the Insurance Institute of Highway Safety (IIHS)2, an independent organization funded by the insurance industry, perform crash tests on most new cars sold in the United States. These crash tests not only make sure the cars sold in the U.S. meet minimum safety requirements, but they also signal to consumers how well each car tested is likely to protect you in a collision.

Though the NHTSA and the IIHS both crash test new cars, they don't conduct the same tests and they don't rate the cars the same way.

How the NHTSA Rates Cars
The 5-Star Safety ratings that you see on new car stickers are generated by the NHTSA. To get to those 5-Star ratings, which include an overall safety rating, a frontal impact, side impact and rollover resistance score, the NHTSA performs several different types of crash tests.

The first is a front crash test. For this test, the vehicle is loaded with sensors and two crash test dummies (one representing an average-sized man and one representing an average-sized woman), and then crashed into a concrete barrier at 35 miles per hour.

For side impact ratings, the NHTSA performs two crash tests. In the side barrier test, a crash test dummy is put in the driver's side front and back seat of the car, and a moving barrier is then crashed into the car at 38.5 miles per hour, which is meant to represent the car getting T-boned at an intersection.

For the second side impact crash test, the side pole test, a dummy is placed in the driver's seat and the car is pulled at a 75-degree angle into a pole at 20 miles per hour, to test how a car might perform if it collided with a pole. From these two side impact crash tests, the NHTSA gives the vehicle an overall side crash test score.

The NHTSA also tests how likely a car or SUV is to rollover during a sudden change in direction. While not exactly a crash test, it still provides valuable safety information to consumers.

The NHTSA also gives higher ratings to cars, trucks, and SUVs that have accident avoidance features like a rearview camera and forward collision warning and lane departure warning systems.

How the IIHS Rates Cars
Like the NHTSA, the IIHS gives several different ratings for new car crash worthiness. A car that has strong performance in each of the five crash tests the IIHS performs, will earn the IIHS Top Safety Pick designation. If a car gets top scores in all crash tests among other vehicles in its class and has front crash prevention features, it earns the Top Safety Pick+ designation.

The IIHS performs two frontal crash tests. The first is the small overlap front test, which measures how well a vehicle protects occupants during a collision on the front of the car — specifically, on the driver's side — as if the nose of the car had crossed over the double yellow line and hit another car or a utility pole or a tree. For this test, the vehicle is crashed into a barrier at 40 miles per hour.

The second front crash test the IIHS performs is the moderate overlap front crash test, in which almost half of the front of the car is involved in the collision — think of a head-on crash between two cars. This test is also conducted at 40 miles per hour.

For the IIHS's side impact crash test, a barrier representing an SUV or crossover is crashed into the side of the car at 31 miles per hour. Like the NHTSA's side impact crash test, this is meant to represent a T-bone crash at an intersection.

While the NHTSA tests rollover resistance, the IIHS tests how likely a car is to protect occupants in a rollover by measuring roof strength. To do it, the IIHS crushes the car's roof in a special machine. The machine measures how much force is needed to collapse the roof by 5 inches. The more force that's required, the stronger the roof is rated.

In contrast to the NHTSA, the IIHS tests how well the vehicle's seats and head restraints protect occupants in a rear-end collision. For this one, they don't crash an entire car. Instead, the seats are put on a sled, a dummy is strapped in, and the sled is then moved to mimic a rear-end collision.

Lastly, the IIHS tests front crash prevention tests, like forward collision warning and automatic braking. For this, they do two tests: one at 12 miles per hour and one at 25. The car is driven at the test speed toward an inflatable barrier. The car is evaluated by how well it slows or prevents the collision altogether.

Don't buy a new car until you read up on the different crash test ratings from @NHTSAgov and @IIHS_autosafety. More details here. []
How You Should Use Crash Safety Ratings
When you're shopping for a new car, you should use the crash safety ratings to compare the protection offered by different cars. You can assume that a five-star car has better crash prevention than a four-star vehicle. However, because the overall score is based on component scores, you really can't tell how much safer a five-star car is versus a four-star one.

At the same time, you also really can't compare scores across different vehicle classes. For the most part, a car is evaluated based on how much protection it offers relative to other similar cars. That means a large SUV's crash test scores can't be compared to a small hatchback's.

As helpful as crash test ratings are, be mindful that even a car with the best ratings from both the IIHS and the NHTSA is only as safe as it's being driven on the road. You'll notice that the top speed of any crash test conducted by the NHTSA and IIHS is just 40 miles per hour, so while it's safe to extrapolate that a five-star car will still give you better protection than a three-star one at 70 miles per hour, we really can't objectively say how much crash protection is provided at those higher speeds. And while these rated crash tests all happen in a controlled environment, that's not the real world. Out here, the safest crash is the one that never happens.
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