How do Cars Get Their NHTSA Star Rating?

Between 1994 and 2010, cars were given a "star rating" by NHTSA, based on the risk of severe injury, "an injury that requires immediate hospitalization and may be life threatening". Star ratings are also applicable to 1979-1993 vehicles using the same formula. Two injury measures were used, HIC (Head Injury Criterion) and Chest G (force applied to the ribcage) to get an indication of the combined injury risk to the head and chest.

5 stars - 10% or less risk. In practice, no or minor injury likely to most people.
4 stars - 11-20% risk. In practice, minor to moderate injury likely to most people.
3 stars - 21-35% risk. In practice, moderate to serious injury likely to most people.

2 stars - 36-45% risk. In practice, serious injury likely to most people.

1 star - 46% or greater risk. In practice, serious, severe, critical, or fatal injury likely depending on HIC and Chest G scores.

These two tables were used by NHTSA to determine star rating:

To determine your risk in a crash, use this formula: Headrisk + (100%-Headrisk * Chestrisk)

Sound confusing? I'll walk you through the formula.

Let's say you want to know the driver's injury risk for the 2010 Mercedes E Class. The driver HIC was 606 and Chest G was 46. If you look at the graph, you find that head injury risk is about 5% and chest injury risk is 9%. Injury risk is thus:

5% + (0.95 * 9%)

5% + 8.55%

Risk of serious injury is about 13.5%, giving the E class a 4 star rating for the driver. A real life driver would likely suffer minor to moderate injury.

If you want to know the risk for your vehicle: you have two options:

1) Use the formula. Go to http://www.safercar.gov/Vehicle+Shoppers/5-Star+Safety+Ratings/1990-2010+Vehicles, search your vehicle, find the test number under the "Frontal Crash" tab (it will be a number in the 5000s or 6000s for newer vehicles, 2000s-4000s for older vehicles), go to the NHTSA crash test database, put in the test number, click on "Vehicle Information" then "Occupant Information". The HIC is under "Head Injury Criterion" and the Chest G is under "3 millisec. clip tdorax region peak acceleration (g).

2) Leave a comment with the car you want to look for on this post, and I will look it up for you and tell you your risk. It's no problem so don't hesitate to leave a comment.