How to Understand Car Safety Ratings

Toco Joe from Toco Warranty explaining car safety ratings

Learn how car safety ratings are determined and how to use them when buying your next vehicle.

Although vehicle safety may not always be the #1 factor you consider when buying a new car, it's something that you should always be aware of before driving off the lot. Thankfully, car safety ratings can help demystify the comparison process. But while these ratings can help you make an informed decision when buying your next vehicle, it's important to understand how these ratings are determined and what they mean for your safety.

Insurance Institute for Highway Safety (IIHS)

The Insurance Institute for Highway Safety, or IIHS, conducts detailed safety tests for every vehicle on the market in the U.S. These tests determine the car safety ratings for each vehicle. You can search for test results for a specific make and model directly through the IIHS website.

There are five principal tests that IIHS analysts use to evaluate vehicles.

Front Crash Tests

Because front-end collisions lead to so many traffic injuries, they're given particular scrutiny when it comes to crash tests.

The IIHS conducts moderate overlap tests and small overlap tests for front crashes. In this case, "overlap" refers to the amount of the front bumper that makes contact with the barrier. They conduct two separate front crash tests to see how the car performs in different situations:

  • In the moderate overlap tests, roughly 40% of the width of the vehicle strikes a barrier head-on at about 40 mph.
  • In the small overlap tests, 25% of the total width of the vehicle strikes a barrier at the same speed. Small overlap front crash tests are meant to simulate things like a crash into a tree or a telephone pole.

The IIHS uses the following ratings criteria for a front crash test:

  • The strength of the structure or safety cage that's supposed to insulate the driver or passenger from much of the impact of the front crash.
  • The extent of injuries, using crash test dummies, and the movement of the dummy in the simulated collision.

The resulting scores indicate how much safer a car is than the average:

  • A “good” rating means a driver or passenger is 46% less likely to die in the crash.
  • A “marginal” or “acceptable” rating means a driver or passenger is 33% less likely to die in the crash.

Side Crash Tests

In side crash tests, a large barrier of 3300 pounds hits the side of the vehicle traveling at 31 mph. This effectively simulates many kinds of crashes where a vehicle might run into another one from the side, such as at an intersection.

Ratings criteria include:

  • Injury measures by looking at the crash test dummies
  • The extent of head protection, using evaluations of head movement during the simulated crash
  • The vehicle’s structure or safety cage

In a side crash test rating:

  • A rating of “good” means a vehicle occupant is 70% more likely to survive.
  • An “acceptable” rating means a vehicle occupant is 64% more likely to survive.
  • A “marginal” rating means a vehicle occupant is 49% more likely to survive.

Roof Strength Test

Since many injuries occur when the driver's vehicle flips over and ends up upside down, IIHS analysts also evaluate vehicles in terms of roof strength.

Vehicles have to be strong enough to maintain cabin structure to help you stay safe. And since many large vehicles have a high center of gravity (which can lead to overturning), roof testing is a critical measure of the safety - especially if you're buying an SUV or truck.

In roof tests, research teams push a metal plate against one side of the roof at a constant speed. This is used to record a strength-to-weight ratio for the vehicle. The IIHS looks at the peak strength-to-weight ratio before the roof is crushed 5 inches.

In a roof strength test:

  • A “good” IIHS rating on roof tests requires a strength-to-weight ratio of at least 4 (which means the vehicle must stand up to a force of at least four times the vehicle’s own weight)
  • An “acceptable” rating, requires a strength-to-weight ratio of 3.25
  • A “marginal” rating requires a ratio of at least 2.5

Restraint Tests

In addition to the front impact, side impact and roof strength tests, the IIHS also tests a vehicle by evaluating head restraints and other types of restraints. In this type of test, analysts looks at how well the seats support the torso, neck and head of crash test dummies.

By simulating a rear end crash with a low speed of around 10 mph, analysts try to judge the car's performance during lower impact accidents that can cause whiplash and other types of injuries.

The head restraint ratings depend on a number of specific criteria, including the amount of torso acceleration and the neck sheer force on a restraint. By combining various ratings, the IIHS issues a final dynamic rating for restraints and seats.

How to Assess IIHS Safety Ratings

The IIHS maintains a full set of top safety pick winners for each model year, right up to the current year. You can look for top safety picks for all vehicle segments, including mini cars, small cars, midsize cars, midsize luxury cars, large cars, large luxury cars, small SUVs, midsize SUVs, midsize luxury SUV's, large SUVs, minivans and large trucks. In addition to searching by category, you can also search by the year, make and model of a specific vehicle.

To view the safety ratings of each vehicle, look for the full series of test indicators, all of which use a system with values of good, acceptable, marginal and poor. These combined ratings are how IIHS assesses which vehicles are safer than others.

If safety is a primary concern, car safety ratings can provide an invaluable tool for making an informed decision.