The Institute has been conducting 40 mph frontal offset crash tests for consumer information since 1995. Auto manufacturers have responded by improving the structures of their passenger vehicles to earn good ratings in the Institute's test. The result has been a measurable improvement in occupant protection in real-world frontal crashes (see "Test performance predicts outcomes in real-world crashes," Feb. 7, 2004).
The 1997 Toyota Trans Sport received a POOR rating.
Now the National Highway Traffic Safety Administration (NHTSA) is requesting comments on possible frontal offset crash test requirements. The Institute believes a federal offset test standard would be a step forward in terms of frontal crash protection. However, in comments submitted to the agency last month the Institute pointed to a number of unsound assumptions NHTSA has made about the potential pros and cons of mandating such tests.
Structural integrity gets short shrift
In proposing to require offset tests, NHTSA said the primary benefit would be vehicle designs that reduce injuries to occupants' legs and feet. The agency based this on the fact that these are the only parts of test dummies where high forces were recorded during research testing.
This narrow focus understates the benefits of other aspects of vehicle design that improve performance in offset tests, the Institute told the agency. Dummy injury measures aren't the only parameters, or even the most important ones, for evaluating a passenger vehicle's performance in a frontal offset test. First and foremost NHTSA should evaluate the structural performances of vehicles.
Good measures recorded on dummies during tests "indicate good frontal crash protection only if the occupant compartment is not significantly damaged. If the compartment is significantly damaged, dummy measures below injury assessment reference values offer no assurance of effective protection" in real-world crashes, the Institute noted.
NHTSA should incorporate a structural assessment into the offset crash test requirements under consideration, the Institute advised, because "the primary factor that has affected the Institute's ratings over the years … is the ability of a vehicle's structure to reduce and limit damage to the occupant compartment."
Compatibility won't be compromised
NHTSA assumes that automakers would need to stiffen some vehicles to meet offset test requirements and that this could increase the risks for occupants in other vehicles. But this is wrong. Manufacturers don't simply stiffen the front ends of their vehicles to perform well in offset tests, the Institute pointed out in its comments. Good performance in offset crashes requires strong, or stiff, compartments and front ends that effectively absorb crash forces. To achieve this the crumple zones need to be less stiff than the compartments.
Institute researchers have observed poor offset performance because vehicles' front ends were too stiff and their compartments too weak. Automakers have improved performance by making the compartments of these vehicles stronger and, in some cases, their front ends less stiff. These kinds of design changes actually enhance compatibility in most crashes. Besides, real-world crash data don't show a pattern of vehicles with good test ratings inflicting more harm when they strike other vehicles.
Institute researchers analyzed driver deaths in frontal crashes involving vehicles that had been evaluated based on offset tests. The main finding is a "remarkably consistent pattern" of higher death rates in both poor-performing vehicles and in other passenger vehicles with which they collide. Tested vehicles with good evaluations have lower death rates for their own occupants and for occupants in the other vehicles with which they collide.
This pattern "contradicts NHTSA's concern that improved frontal offset test performance might be leading to increased aggressivity in crashes with other vehicles," the Institute told the agency. Institute chief operating officer Adrian Lund adds that "it doesn't make sense to assume that good vehicle compatibility in crashes requires some passenger vehicles to have weak occupant compartments that are inadequate to protect their own occupants in serious frontal crashes."
NHTSA's ill-conceived tests
The agency reached its conclusions about the potentially adverse consequences based on a series of tests in which vehicles rated by the Institute for frontal offset performance struck Honda Accords. The results of these tests (high-speed, 30-degree, oblique angle impacts) were compared for pairs of striking vehicles before and after design changes were made to improve performance in the Institute's offset test. For example, results for a Chevrolet TrailBlazer into an Accord were compared with results for a predecessor Blazer into an Accord.
Injury measures recorded on dummies in the cars struck by the redesigned vehicles often were higher than when the striking vehicles were predecessor models. The agency's conclusion was that the outcomes were due to design changes made to improve ratings in the Institute's test.
The Institute responded by telling NHTSA that its tests were "ill-conceived and inadequately analyzed." Damage to the Accords in the impacts with the TrailBlazer and Blazer, for example, was so extensive that meaningful interpretations of the dummy injury measures weren't possible. The test configuration is closer to a side impact than a frontal one, and in both tests there was essentially total destruction of the Accords' occupant compartments. Some of the differences in the head injury measures appear to be due to the somewhat random way the occupant compartments of the Accords collapsed.
NHTSA's tests also "confounded changes in vehicle stiffness with changes in other important vehicle characteristics. No attempt was made in these tests to isolate the unique effects of front-end stiffness," the Institute pointed out. For example, the TrailBlazer is not only stiffer but also heavier and higher than the Blazer. Any of these factors alone or in combination could account for the higher injury measures on the dummies in the Accords.
"NHTSA should go back and re-analyze the potential costs and benefits of offset tests. This should lead the agency to go ahead with rulemaking to include such tests in federal safety standards. Major benefits would be to lock in the crashworthiness improvements that offset testing for consumer information has spurred and to spread the improvements to vehicles the Institute hasn't evaluated," Lund concludes.