Understanding Crash Test Scores

While cars are safer than ever, there are still about 30,000 people who die in automobile collisions every year, so understanding how your car performs in crash tests is the best way to predict what will happen to your vehicle and its occupants should an accident occur. The test scores are a useful tool to help make a purchase decision when you are choosing between several cars, though it's important to remember that scores for frontal and side impact tests can only be compared between vehicles of a similar weight.

There are two agencies that provide crash test ratings. The National Highway Traffic Safety Administration (NHTSA) is the federal agency that provides frontal, side and rollover ratings, giving vehicles a rating of one (the worst) to five stars (the best) based on performance. The Insurance Institute for Highway Safety (IIHS) is a non-profit group backed by auto insurers that conducts frontal and side crash tests, roof strength tests and head restraint tests. It rates cars from "poor," "marginal," "acceptable" and "good" and also selects cars that perform well on both frontal and side tests with a "Top Safety Pick" rating.

While both groups conduct similar types of testing, each performs its tests using different methods, so it's wise to look at both sets of scores to get a complete picture of how a certain car performs in a crash. Each year, both groups choose cars to test based on their popularity with buyers that have structural or safety changes from the prior model year. These new tests, combined with carry-over ratings from vehicles that are largely unchanged from prior model years, result in ratings for the majority of vehicles for the current model year.

Frontal Impact Ratings
In NHTSA's frontal tests, cars are crashed into a fixed barrier at 35 mph, which represents a head-on collision of two cars of similar size and weight each moving at 35 mph. Instruments measure the impact to multiple areas of the crash-test dummies and the star ratings are given based on the chance of serious injury--one that requires hospitalization and may be life threatening. Beginning with the 2010 model year, NHTSA's frontal crash test program will use new test dummies and different injury criteria. As a result, ratings for 2009 model year cars and earlier will not be comparable to the ratings beginning with 2010 model year cars.

The frontal crash test that IIHS performs is an offset test, with the car crashed at 45 mph into a fixed barrier at an angle (instead of head on). The test is designed to simulate two cars of similar weights colliding so about 40 percent of each car is impacted. This test complements NHTSA's test and assesses the integrity of the passenger compartment whereas NHTSA's test assesses the effectiveness of the restraint system.

Side Impact Ratings
Both group's side crash tests are designed to simulate a collision in an intersection where the car is hit in the driver's side by another vehicle. NHTSA uses a 3,015 pound deformable barrier moving at 38.5 mph, whereas the IIHS uses a 3,300 pound deformable barrier in the shape of the front end of an SUV traveling at 31 mph. The biggest difference is that NHTSA's test uses average-size male dummies, while IIHS uses dummies the size of a very small adult woman, which is also considered equivalent to an average-size 12 year old. Real-world data indicates that smaller people are more likely to experience severe head injury in a side impact crash because their shorter stature means their head is in line with the window, making them more vulnerable.

Beginning with 2010 model year vehicles, NHTSA will begin using new test dummies and different injury criteria for the side impact test, so ratings will not be comparable to previous model years. The agency will also add a new test that simulates a side impact into a pole.

Rollover Ratings
Currently NHTSA calculates its rollover ratings for vehicles based on a static test that is essentially a mathematical calculation and a dynamic test that simulates a vehicle that swerves suddenly, which often starts a chain reaction that leads to a rollover. This is the only test where consumers can compare cars of all sizes and types against each other.

The IIHS does not conduct rollover crash tests but it now offers a roof strength rating system. The new system is the result of research that indicates that stronger roofs equate to the occupants' reduced injury and chance of ejection in a rollover crash. The test is conducted with a metal plate that is pushed into one side of the roof at a constant speed. In order to receive the IIHS' best rating of "good," it must withstand four times the force of the car's weight before crushing down five inches. "Our research shows that a strength-to-weight ratio of 4 reflects an estimated 50 percent reduction in the risk of serious and fatal injury in single-vehicle rollover crashes compared with the current federal standard of 1.5," said Joe Nolan, vice president at the IIHS.

In its initial round of tests, the IIHS assessed 12 small SUVs from the 2008-2009 model years and it will continue to test additional vehicles. Beginning with 2010 models, a vehicle will need to earn a "good" rating in roof strength in order to receive the Institute's Top Safety Pick award.

Crash test scores from both NHTSA and IIHS are designed to give drivers a picture of how their car will perform in common types of collisions. A low score does not mean that your car is unsafe--it simply means that it did not test as well as other cars of similar size and type.

By Tara Baukus Mello