Interpreting Crash Tests and Safety Ratings

Overall, passenger vehicles are safer than they've ever been, but the crashworthiness of individual models varies greatly, even within a vehicle class. New roof-strength tests also reveal that one version of a model can fare better than another. Because of this, understanding how a model is expected to perform in a crash is important before buying your next car.

The most important things to know about crash tests:

Not All Results Can Be Compared
Crash Tests Differ by Agency
NHTSA Star-Rating System Revamped
Side-Impact Tests Differ
Government Rollover Ratings Have Shortcomings
Roof-Strength Tests Provide Key Rollover-Protection Data
Some Models Are Not Rated

Not All Results Can Be Compared

Model-to-model comparisons of frontal crash ratings are valid only within a vehicle class or between models of comparable weight (as long as they're within 250 pounds of each other). The test reflects how the vehicle would fare in a collision with another of the same model, not versus a larger or smaller vehicle (or a lower- or higher-riding vehicle). A heavier vehicle protects its occupants better than a lighter one if all other factors are equal, but they almost never are. So a large vehicle with a Poor rating is not necessarily safer than a small vehicle with a Good rating. Unfortunately, researchers have not yet devised a reliable method for reporting the effect of size differences on a vehicle's score.

Note: Side-impact crash tests are comparable across classes because the sled that rams the test vehicles is of a consistent size and weight.

Likewise, the Insurance Institute for Highway Safety's rear-crash head-restraint ratings consistently test how well a stationary seat protects against whiplash by simulating a 20-mph rear crash. IIHS combines the results with an evaluation of the seat's geometry to arrive at a rating. Oftentimes vehicles offer more than one type of seat
say, manual and powered and they don't perform the same in crash tests. Shoppers are wise to scrutinize the ratings to determine which seats have scored well. Back to top

Crash Tests Differ by Agency

There are two testing agencies, and they perform different types of frontal tests. The National Highway Traffic Safety Administration crashes cars head-on into a solid, immovable barrier. Neither the angle nor the obstacle corresponds with the majority of real collisions. IIHS conducts a frontal-offset crash into a deformable barrier that reacts like another vehicle. This gauges how well half of the vehicle's front end absorbs crash energy. Many experts say this test is more revealing and better represents the majority of real-world crashes. Back to top

NHTSA Star-Rating System Revamped

Starting with the 2011 model year, the testing landscape changed, thanks to an overhaul of the National Highway Traffic Safety Administration's five-star New Car Assessment Program. The changes range from a new side-pole test to the use of a wider range of test-dummy sizes that better represents occupants of all sizes. The new regimen also records more data, which is then analyzed and weighted to produce a single overall safety rating to accompany the existing separate frontal- and side-impact crash-test ratings and rollover-resistance ratings.

The casualty of NCAP's overdue modernization is that a majority of the market has not been tested. Because the methodology has changed so much, 2011 and 2012 results can't be compared with earlier scores, which Cars.com gives little credence. Back to top

Side-Impact Tests Differ

NHTSA remedied a shortcoming of its earlier side-impact crash tests by factoring head-injury data into the side rating, which now combines the side barrier test with a pole test that simulates a car sliding sideways into a tree or post. This test can reveal weaknesses that aren't reflected in the traditional barrier test, in which the test car is T-boned by a sled.

The sled is roughly the weight and height of a midsize sedan. The IIHS side-impact test   employs a sled as high and heavy as a full-size SUV or pickup, creating a more dangerous scenario. In either test, the sled's size is consistent, so comparisons of side-impact ratings are valid across vehicle classes. Because of the different methodologies, both the IIHS and the NHTSA tests are useful in determining a car's crashworthiness.

It's important to scrutinize crash-test reports
not just to determine if the car has side airbags, but to know if they are standard or optional on the car you're considering. In some cars, side bags have meant the difference between getting a top or a bottom score, and it's up to you to make sure the car you buy has them. Side-impact tests use properly positioned, belted test dummies, which doesn't tell us what would happen if an occupant were out of position in which case the side airbag firing can itself be hazardous, especially for children.

To address this, NHTSA reports have an "SAB Out Of Position Testing" field that may read "Meets Specs." You should know that this result is being reported by the automaker after voluntary testing; it's not a test performed by NHTSA. People concerned about injuries from side airbags should avoid the seat- or door-mounted type, or buy a car that disables those airbags when the occupant is out of position. Honda and Acura have pioneered this feature. Curtain airbags are considered to be less dangerous. Back to top

Government Rollover Ratings Have Shortcomings

Auto manufacturers and safety experts considered NHTSA's original Rollover Resistance Ratings, begun in the 2001 model year, inadequate at judging a model's rollover propensity because they were based on a mathematical calculation of the vehicle's center of gravity. Starting with the 2004 model year, NHTSA combined this calculation with a "fishhook" dynamic driving test in which the test vehicle swerves suddenly and then overcorrects. The combined results, NHTSA Rollover Ratings, give a percentage chance of rollover a star rating based on this chance and whether the model tipped up on two wheels during the fishhook test or not. While many see this as a step in the right direction, some automakers still criticize NHTSA for extrapolating some conclusions. Back to top

Roof-Strength Tests Provide Key Rollover-Protection Data

Where NHTSA attempts to relate a model's propensity to roll over, roof-strength tests from IIHS reflect how the roof might protect occupants when a rollover occurs. Using its familiar scale Good, Acceptable, Marginal, Poor IIHS in 2010 began to rate models based on how well they resist up to four times their weight in a crush test. Because weight varies among different versions of the same model, two- and four-wheel-drive versions of the same model could earn different scores. The extra weight of hybrid hardware has earned the hybrid version of Ford's Escape a Poor roof-strength rating even though the non-hybrid is rated Marginal. For more information on roof strength as it relates to safety, see the related Roof-Strength Ratings Offer Insight on Rollover Safety. Back to top

Some Models Are Not Rated

If the model you seek is missing crash-test results, they may be pending or the vehicle may not be tested. This situation worsened when NHTSA revamped its tests for 2011, essentially discarding ratings for the majority of models. Both agencies concentrate on the highest-volume vehicles. Convertibles are rarely tested for this reason, though for the first time in 2007 IIHS tested 10 models, including several best-sellers like the Chrysler Sebring and Ford Mustang. Results for new or recently re-engineered models are likely to appear months after the car goes on sale because both agencies purchase their test subjects from dealerships. NHTSA notes if a vehicle is TBT (to be tested) or if results are pending or under review. IIHS has begun to offer more detailed information about whether test results are pending. Back to top

© Cars.com 02/18/2014