How NHTSA Ratings Work

Buying and selling cars can be a pretty subjective process. Car buying can come down to what the buyer likes, so a large part of car selling is about figuring out what that might be. But there's one part of buying cars -- and selling cars -- that's as objective as possible: safety ratings.

By law, every car on the U.S. market has to meet certain safety standards set by the federal government. But most car buyers want cars that go beyond those standards. Short of crashing on your test drive, how can you determine if the car you're interested in is safe?

You really can't. Sure, cars can be stuffed with airbags and safety systems, but until those systems are called upon to prevent crashes and protect occupants, you really can't know how well they work. Yet, when you're buying a car, how those systems work is one of the most important and least subjective parts of the buying process. Car buyers simply want to know how safe a car is.

That's where safety ratings come in. Safety ratings are the results from crash tests, which show, as scientifically as possible, how a car's safety systems perform during collisions. In some cases, the ratings also show how adept the systems are at preventing a crash. While there are private organizations that give safety ratings, ratings from the National Highway Traffic Safety Administration (NHTSA) are required before a car can be sold in the Unites States and those ratings must be displayed on a car's window sticker.

National Highway Traffic Safety Administration

It's a little-heard-of government agency, but the National Highway Traffic Safety Administration is a key part of car buying. Before a car can be sold in the United States, it has to undergo NHTSA testing. Good NHTSA ratings can lead to the automaker selling cars -- in fact, many more cars than a poorly-rated model sells. When a model gets good safety ratings, you'll hear about it in print, radio and television ads, and at the dealership, too. When it comes to buying cars, highly-rated models tend to be more popular with consumers, making the NHTSA and integral part of both car buying and selling.

So, what exactly is the NHTSA? The NHTSA was established in 1970. Its mission is to "save lives, prevent injuries and reduce economic costs due to road traffic crashes, though education, research, safety standards and enforcement activity." The NHTSA's task is crucial: Automotive crashes are the leading cause of death across all age groups for Americans and take an estimated $230 billion from the economy each year [source: NHTSA].

While NHTSA has a number of programs to educate drivers and communities about road safety, the agency is best known for safety recalls and vehicle product testing. In both cases, the NHTSA tests and recalls not only cars, but other products used with cars, like child safety seats.

If you think about it, it's easy to see how important the work the NHTSA does actually is for consumers. Without the NHTSA, there wouldn't be a system to track safety issues with cars and force car makers to recall (and fix) unsafe models. Also, parents would have no way of knowing if the car seats they put their kids in will truly keep them safe. Without NHTSA-developed safety standards, car makers could put unsafe cars on the road, too. And finally, without NHTSA ratings, car buyers wouldn't know if the car they're buying is actually safe.

How NHTSA Ratings are Determined

Crash-test ratings from the NHTSA are a key part of car buying and car selling. Anyone selling cars wants good ratings they can brag about, while anyone buying cars wants to steer clear of lower-rated models. When it comes to buying and selling cars, NHTSA ratings can mean big business.

That's why the NHTSA tries to take the business out of its ratings as much as possible. New car crash testing falls under the NHTSA's New Car Assessment Program (NCAP). As part of the program, new cars are crash tested under similar conditions, so consumers can easily compare the data. As part of the program, the NHTSA performs front and side impact crash tests, as well as rollover tests, to give its ratings.

Front crash tests are performed by crashing a car into a barrier. The goal is to simulate a collision between two similarly-sized cars, each traveling at 35 miles per hour (56.3 kilometers per hour). It's important for consumers to remember that when it comes to ratings, they only show how the car might perform in a crash with a car that's the same size, not crashes in general. Remember, if you're buying a car that's smaller than anything on the road, the crash test ratings don't necessarily apply if you crash into a much larger vehicle. On the plus side, since consumers tend to shop for cars by size, it's easy to see how the cars you're considering compare, since each is essentially crashed into itself.

Side-impact crash tests use a barrier that smashes into the side of the car. The test simulates a crash between two similarly sized cars, one traveling at 34 miles per hour (54.7 kilometers per hour) and the other traveling at 17 miles per hour (27.4 kilometers per hour). Again, the results only apply to collisions between cars of similar sizes, but they do provide a standard comparison for car shoppers.

The NHTSA also gives roll over ratings to each car. Those ratings are created through what the NHTSA calls "dynamic vehicle testing," where NHTSA tests how stable the car is by quickly changing directions and also by measuring the height of the car's center of gravity and how wide its track is. As a general rule, cars with a wide-track and a low center of gravity are less likely to roll over than tall, thin vehicles.

Types of NHTSA Ratings

After testing the cars, the NHTSA releases its ratings. For front- and side-impact crash tests, the NHTSA releases two ratings. In both tests, there's a driver safety rating. That rating reflects how well the vehicle protects drivers in either a front or side impact.

When it comes to passengers, the ratings differ a little. For front-impact crash tests, the NHTSA gives a rating that reflects how well the vehicle protects the front passenger. However, no rating is given to how well the vehicle protects rear passengers in a front-impact crash test. That's because front-seat passengers take the brunt of the collision in front crashes.

In side-impact tests, the collision is on the driver's side of the car. As a result, no rating is given to front passenger protection in side-impact crash tests, but there is a rating given for rear-seat passenger protection (since the passenger behind the driver would be exposed to most of the crash force, like the driver).

In all crash-test ratings, the vehicle is given stars for how well it protects the driver and affected passenger. The rating scale runs up to five stars, with the vast majority of cars getting at least three stars in all tests.

Roll over ratings don't reflect how well a vehicle protects occupants in an accident. Rather, roll over ratings reflect how likely a vehicle is to roll over in a single-vehicle accident. Roll over ratings also use stars, but the rating is on a four star scale, and each star reflects the chance a vehicle could roll over. For example, the 2010 Chevrolet Tahoe has a three-star roll over rating, which means it has about a 24-percent chance of rolling over in a single vehicle accident.

If you're interested in NHTSA ratings, you can check them out by visiting safercar.gov, where you can search the NHTSA's database.

Improving NHTSA Ratings

Now you know how NHTSA ratings work, and why they're so important for car buying and car selling. But, when it comes to buying and selling cars, are NHTSA ratings the last word when it comes to safety?

Not really. NHTSA ratings are extremely helpful, but if you're really concerned about safety when you're buying a new car, you'll probably want to look beyond NHTSA ratings. One good place to look is the Insurance Institute for Highway Safety (IIHS). The IIHS also crash tests and rates cars, but its ratings are different -- and in some cases, more strict -- than NHTSA ratings. For example, the NHTSA doesn't give ratings on how a car might protect its occupants in rear-end collisions, and the IIHS does. Also, when it comes to rollover crashes, the NHTSA only rates how likely a car is to rollover. The IIHS performs roof-strength tests. Stronger roofs are key to protecting everyone in a car that's rolling over.

Finally, while both the NHTSA and the IIHS typically perform crash test that simulate collisions between two identically-sized vehicles, the IIHS occasionally goes beyond that. Noting the trend toward extra-small cars, in the summer of 2009, the IIHS performed crash tests that crashed small cars (the Toyota Yaris, Honda Fit and smart fortwo) into midsized cars -- with surprising results. Cars that rated well when crashed into similarly-sized vehicles didn't do so well when crashed into larger vehicles. That's information a lot of consumers would like to know.

But, the NHTSA is constantly refining and updating its ratings and crash tests. For example, the rollover rating was developed in response to the number of SUVs on the road and the increasing number of rollover accidents. By adding the rollover rating, the NHTSA gave consumers the information they needed to make the best car purchase for them.

For more information about NHTSA ratings and other related topics, follow the links on the next page.