A 2008 Nissan Pathfinder gets "T-boned" in a side-impact crash test by the Insurance Institute for Highway Safety.
(Associated Press)
Overall, passenger vehicles are safer than they've ever been, but the crashworthiness of individual models varies greatly, even within a vehicle class. Because of this, understanding how a model performs when involved in a crash is important before buying.
All vehicles sold in the United States must pass Federal Motor Vehicle Safety Standards in the form of 30-mile-per-hour frontal and 33.5-mile-per-hour side-impact crash tests, but the relative safety of all vehicles above this minimum varies greatly.
Model-to-model comparisons of frontal crash-test ratings are valid only within a vehicle class or between models of comparable weight (within 250 pounds). The test reflects how the vehicle would fare in a crash with another of the same model, and not versus a larger or smaller vehicle (or a lower- or higher-riding vehicle). A heavier vehicle would protect its occupants better than a lighter one if all other factors were equal, but they never are. So a large vehicle with a Poor rating is not necessarily safer than a small vehicle with a Good rating. Researchers have not yet devised a reliable method for reporting the effect of size differences on a vehicle's score.
Side-impact crash tests are comparable across classes because the sled that rams the test vehicles is of a consistent size and weight.
The testing agencies 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. The Insurance Institute for Highway Safety conducts a frontal-offset crash into a crushable barrier that reacts like another vehicle. This test 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.
Side-impact crash tests currently aren't as telling as some would like. Though NHTSA has tested more models than IIHS for side-impact protection, these tests are criticized for two reasons:
+ The sled used to "T-bone" the stationary test vehicle has the height and mass of a car, not a sport-utility vehicle or pickup truck. This tends to minimize its intrusion into the cabin - a best-case scenario.
+ The NHTSA's chance-of-injury data are based on trauma to the test dummies' torsos, not their heads. Occupants' heads are more susceptible to injury in a side impact, and head injuries are more often serious and potentially fatal.
The IIHS side-impact test does measure head injury and employs a sled as high and heavy as a full-size SUV or pickup, a more dangerous scenario. Unfortunately, IIHS has just begun doing this and few models have been tested. Because the sled is consistent, comparisons of side-impact ratings are valid across vehicle classes.
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 solely 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, called simply NHTSA Rollover Ratings, give a percentage chance of rollover - a star rating based on this chance and whether or not the model tipped up on two wheels during the fishhook test. These new ratings are steps in the right direction, though some automakers still criticize them for extrapolating some conclusions.
Convertibles are rarely tested. Results for new or recently reengineered models are likely to appear months after the vehicle goes on sale because both agencies purchase their test subjects from dealerships, just as consumers do.
© Copyright 2008 Globe Newspaper Company.
