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Crash test 101

How crash worthiness is measured and how crash ratings can help you choose your next car

Published: April 2014

Small-overlap test
The Insurance Institute for Highway Safety (IIHS) tests a Chevrolet Spark.

 

 

Although vehicles in general are much safer in collisions than they used to be, more than 20,000 people traveling in passenger vehicles still die in crashes every year. Many factors contribute to fatal crashes, including hazardous driving, failure to wear safety belts, poor road conditions, and the vehicle's crash-avoidance capabilities. But, the actual vehicle you're sitting in when a crash occurs can make a life-or-death difference.

Crash tests provide insight into the protection offered by the vehicle itself. As a secondary benefit, the published crash ratings encourage automakers to make ongoing improvements. But with two primary testing organizations (government and insurance industry), multiple tests conducted on each car, and competing manufacturer claims, it can be difficult to make sense of it all. This crash-test primer will enable you to understand the information that matters most.

Structural design and safety systems determine how well a vehicle protects its occupants. But it is only independent crash testing under controlled conditions that differentiates one car from another and tells us how well its key components work together. A crash test may reduce the vehicle to a shattered wreck, yet good structural design keeps passenger-space intrusion to a minimum. Important safety systems such as safety belts, air bags, and head restraints serve a vital role, by restraining, positioning, and cushioning occupants while a collision takes place.

The two independent crash-test information sources are the National Highway Traffic Safety Administration (NHTSA), a branch of the Department of Transportation, and the Insurance Institute for Highway Safety (IIHS), an independent safety-research group sponsored by auto insurers. Using different methodologies, both organizations conduct front and side-impact crash tests. In addition, NHTSA tests for rollover propensity and the IIHS evaluates rear-crash protection and roof strength.

See videos of how cars perform in IIHS' front and side crash tests.

NHTSA crash tests

As part of its New Car Assessment Program, NHTSA scores its tests using a scale of one to five stars; the more the stars, the lesser the likelihood of injury or death. The IIHS uses a four-level scale: Poor, Marginal, Acceptable, and Good.

NHTSA reforms its star ratings

Starting in late 2010, NHTSA overhauled the way it conducts and scores crash tests. The changes are profound enough that the new star ratings for 2011 and subsequent models are not comparable to those assigned to 2010 and earlier models.

NHTSA's new star-rating system should provide better information. Under the old system, most vehicles were racking up four or five stars in every category. The new system intends to be more discriminating.

Some cars that had been earning five stars earned only three under the new system. That's because NHTSA is now factoring in more injury parameters, has added more tests, and is including data from dummies representing a small adult female instead of just an average-sized adult male.

NHTSA has also fundamentally changed the way it assigns the star ratings. Whereas under the old system the scores were based on a calculation of likelihood of serious injury, the new system will compare cars with each other. So it won't be enough for, say, a car to provide good head protection. To get a top score it will now have to provide better head protection than most other cars.

Key components of NHTSA's post-2010 crash-ratings:

  • NHTSA assigns a single overall safety score that combines the results from front, side, and rollover tests. Front-crash results will weigh heaviest in the overall score.
  • The 35-mph full-frontal crash test uses a new 5th-percentile (small adult female) dummy instead of a 50th-percentile "male" dummy on the passenger side.
  • Additional measures for chest deflection, neck extension, and femur and foot injuries were added to the front-crash score.
  • The side-impact crash includes data from the head, chest, abdomen, and pelvis, instead of just the chest. The rear passenger is a 5th-percentile female dummy, instead of a 50th-percentile adult male, and include data from the head and pelvis.
  • A sideways-into-pole test was added, using the small adult female dummy.

Front crash

NHTSA's front-crash test accelerates a car straight into a rigid barrier at 35 mph, with the entire width of a vehicle's front end hitting the barrier. Instrument-bearing, seat-belted crash-test dummies in the two front seats record the level of crash forces on the head, neck, chest, and legs. Those measurements correlate with injury, but formerly only the head and chest results formed the basis of the star rating. Individual star ratings are assigned to the driver and the front passenger. Some automotive experts have criticized NHTSA's full-frontal, rigid-barrier test as unrealistic because such head-on crashes into a flat, solid wall are relatively rare. Others argue that real-world or not, flat-barrier testing is a good way to gauge the effectiveness of the restraint systems, primarily the safety belts and air bags.

Side crash

NHTSA's side-impact test represents a vehicle struck on the left side by a 3,015-pound car traveling at 38.5 mph. Such a scenario mimics what could happen if you were hit on the side at an intersection. Individual side-impact star-rating scores are assigned to the driver and left-rear passenger. For pre-2011 models, only a chest-injury measure dictated the score. For 2011 and later models, the score factors in head, chest, abdomen, and pelvis data for the driver and injury to the head and pelvis for the rear-seat passenger.

IIHS crash tests

Frontal

The Insurance Institute for Highway Safety (IIHS) front-crash tests are quite different from NHTSA's in that they’re designed to highlight the vehicle's structural integrity, as well as restraint performance. IIHS now conducts two series of front-crash tests, one that engages 40 percent of a vehicle’s front and a newer test, inaugurated in 2012, that uses a smaller overlap, engaging just 25 percent of the car’s front.

Both simulate what would happen if two cars of the same weight and type crashed head-on, partially overlapping. The older test, with the 40-percent offset, engages the portion straight ahead of the driver. The newer test is more like a head-on crash where two cars hit left-headlight to left-headlight or a single-vehicle crash into a fixed object like a utility pole or tree. These are very different from the  full-width crash NHTSA uses. Both the IIHS front-crash scenarios use an impact speed of 40 mph instead of 35 mph; and only the left front of the car hits the barrier. The 40-percent overlap test uses a deformable barrier while the 25-percent overlap test uses a rigid barrier.

One effect of the new small-overlap test is that the vehicle tends to rotate around the point of impact as the crash proceeds. Since occupants then move to the side as well as forwards, the test poses new challenges to some safety-belt and air-bag systems. Even though this is a frontal crash, the side-impact air bags may need to deploy as well. Moreover, many cars are not designed to withstand a corner hit as well as they handle an impact that engages a wider portion of the front. There can be more intrusion into the driver’s foot-well, which can cause severe leg injuries.

Both of the IIHS frontal tests are more stringent than NHTSA's because the speed is higher and the crash energy is concentrated on a smaller area. In both, an instrument-equipped crash dummy in the driver's seat records forces to the head and neck, chest, legs, and feet. Vehicles are rated as Good, Acceptable, Marginal, or Poor based on what happens to vehicle structure, as well as forces on the dummies.

Side

The IIHS side-impact test is more severe than NHTSA's. The test uses a heavier striking barrier at 3,300 pounds, compared with NHTSA's at 3,015 pounds. Further, the IIHS barrier strikes higher up on the tested vehicle to simulate a car being hit on the side at 90 degrees by a typical-height SUV or truck. The IIHS bases its scores on head, neck, chest, abdomen, pelvis, and leg injury.

The two dummies in the IIHS side-crash test represent a small adult female or a 12-year-old adolescent. One is the driver, the other a left-rear passenger. Other crash tests performed by NHTSA and the IIHS use a dummy that simulates an average-sized adult male.

See videos of how cars perform in IIHS' front and side crash tests.

IIHS rear-impact evaluations

Though common, not many rear-impact crashes are fatal. But they do cause many injuries, especially whiplash trauma to the neck. The IIHS evaluates rear impacts with physical inspections and crash testing. The crash test, which is conducted with the vehicle seat attached to a moving sled, simulates a rear-end crash about equivalent to a stationary vehicle being struck at 20 mph by a vehicle of the same weight.

The key to rear-impact protection is head-restraint design. Restraints need to be high enough and positioned close enough to the back of the head to cradle an occupant's head in a rear collision. Those restraints that are clearly too low or ill-designed automatically receive a Poor rating from IIHS, while those with a chance of providing decent protection are crash-tested.

What the stars don’t say

Crash tests are useful for gauging how well a vehicle can protect occupants in a crash, but no test is infallible or universal. For instance, most tests use a 50th-percentile (average sized) crash-test dummy, and people much smaller or larger than that may not be protected as well as the scores indicate. Here are some other factors that affect how you should view the scores:

Heavy vs. light vehicles

Since the front-crash tests performed by NHTSA and IIHS simulate a collision between two vehicles of the same weight and height, the scores don't apply to crashes between mismatched vehicles. In a crash between a big car and a small one, you're usually better off in the big car. In such cases, the larger, heavier vehicle projects more of its crash energy into the smaller one. This, in turn, helps to better protect the larger vehicle's occupants, but it can inflict proportionately more injury to the occupants of the smaller vehicle.

Higher bumpers

Besides their weight, the higher bumper on many taller vehicles such as pickups and SUVs contributes to the truck vs. car mismatch. When an SUV or truck hits a typical passenger car, the impact occurs above the car's bumper line, exerting its force into weaker portions of the smaller vehicle and inflicting greater damage. To address this, SUVs—especially car-based models—are being designed with lower, more compatible bumpers.

More on the side

Side-impact tests apply more broadly than front-crash results do. Since the striking vehicle is the same within all the NHTSA tests and within all the IIHS tests, the results apply across all classes. In other words, a Good side-crash score for a small car is the same as a Good for a large car. That means that the side-crash results can be compared across all vehicle-size categories.

How to choose

When shopping for any new vehicle, choose one with good front-and side- crash ratings within its class. Most classes of vehicle offer many good choices, so usually you needn't compromise on safety.

The vast majority of pre-2011 cars earned four or five NHTSA stars for front and side impact. Such a narrow performance spectrum limited the usefulness of the ratings themselves. The IIHS's more-demanding crash tests can better help to narrow down your car choices. Other things being equal, choose a vehicle that the IIHS rates Good or Acceptable in each of its tests.

As crash-test results show, side-impact air bags are an essential safety feature, and side air bags that protect the head and the chest are preferable to those that protect only the chest.

Ultimately, safety is active and passive, balancing the ability to avoid an accident and to survive one. In addition to air bags, belts, and structural integrity, studies have shown that electronic stability control (ESC) is another effective life-saving technology. Starting with the 2012 model year, ESC became standard on all passenger vehicles. When shopping used, you need to be diligent to ensure the specific model you are considering is equipped with ESC.

   

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