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.