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The 2004 Mazda Miata has a five-star rollover rating, meaning it has less than a 10 percent chance of rolling over in a single-vehicle crash.

But just how much greater is the risk of a rollover for that SUV, van or truck? This question and efforts to quantify and publicize rollover tendencies of different vehicles has been debated for some two decades. And it's at the heart of new federal government crash testing.

Starting in 2004, the National Highway Traffic Safety Administration amended its rollover rating system for vehicles to include a dynamic test on a test course. Previously, the government agency had relied on a mathematical formula to compute how likely each vehicle was to roll over.

Science and Behavior Play a Role
While SUVs, trucks and vans tend to get the most attention concerning rollovers, it's important to note that all vehicles, even low-riding cars, can be made to roll over if dangerous circumstances are present. Most rollovers happen when a vehicle goes off the road, perhaps by dropping a couple wheels off the pavement and the driver over-correcting. Sometimes, the vehicle rolls after it strikes a ditch or other object that causes it to begin to tip and roll.

Safety experts agree the rollover risk tends to be higher in tall-riding, narrower vehicles because the center of gravity is higher in these kinds of vehicles. But center of gravity isn't the only thing that affects the potential for a rollover. A driver's behavior and even road conditions can create rollover problems.

But interestingly, interaction with other vehicles isn't necessary in order for a rollover to occur. Federal crash statistics, in fact, show that 85 percent of all rollover fatality crashes are single-vehicle events.

Different Kind of Test Results
NHTSA conducts many crash tests of new vehicles. Two—the frontal crash test and the side-impact crash test—provide star ratings that tell consumers what the likelihood of serious injury is from a crash.

But NHTSA's rollover rating, which also uses a star system ranging from one to five stars, is different. It doesn't indicate the likelihood that occupants will be injured in a rollover; it indicates how likely a vehicle is to roll over in the first place.

For example, one star out of five means a vehicle has more than a 40 percent risk of rolling over in a one-vehicle crash. Five out of five stars means a vehicle's risk of rolling over is less than 10 percent.

The 2004 Explorer Sport Trac 4X2 is one vehicle that received a two-star rollover rating from NHTSA, meaning NHTSA has determined its rollover risk is between 30 percent and 40 percent. The 2004 Miata has a five-star rollover rating, meaning it has less than a 10 percent chance of rolling over in a single-vehicle crash.

Note that NHTSA's frontal and side crash tests have been around longer than the rollover ratings, which began with 2001 model year vehicles. NHTSA began rollover ratings after widely publicized stories about [[%LNKFord Explorers|VipModelYearSelMM|Ford|Explorer|news%] rolling over and killing and injuring passengers after suffering tire blowouts.

Specifics of the Rollover Rating
NHTSA has, for a few years, reported rollover ratings that were derived mathematically.

Called the Static Stability Factor (SSF), it essentially used each vehicle's geometric properties—track width divided by two times the height of the center of gravity—to come up with a number describing how top heavy the vehicle is and thus how likely it is to roll over in a single vehicle crash.

In the new and improved rollover rating that started for the 2004 model year, the SSF is retained and combined with the results of a dynamic maneuver test done by computer-controlled vehicles on a road course.

The vehicle is heavily loaded, as if it has five passengers and a full tank of fuel, and then simulates a high-speed avoidance maneuver of steering one way sharply, then the other way sharply in very quick succession. If both inside tires lift at least 2 inches off the ground during the maneuver, researchers consider the vehicle to have tipped upward.

Other Factors
As mentioned, the rollover ratings are not indicative of injuries likely to be sustained in a rollover crash, just the likelihood that a rollover may occur in a single-vehicle incident.

Yet, over the years, more and more SUVs, vans and even pickup trucks are adding more safety features designed to keep passengers from injury during a rollover. Key among them: Ceiling-mounted curtain or tubular airbags that are programmed to deploy not just in side-impact crashes but during rollovers, too.

In fact, if connected to rollover sensors, these ceiling-mounted airbags typically remain deployed longer than they would in a side crash so they provide protection through the entire rollover event. Among the benefits: They can cover open windows and thus prevent passengers from being ejected during a violent rollover.

Some automakers have even made curtain airbags standard on their SUVs. The Volvo XC90, all Lexus SUVs and the Mercedes-Benz M-Class SUV are among the vehicles with standard curtain airbags. Curtain airbags also are becoming more commonly seen in vans, where they're usually options as they are in the Ford Freestar and Mercury Monterey. Newer pickup trucks, such as the Nissan Titan, also offer curtain airbags as options.

Automakers also are adding stability control systems to many vehicles to help drivers maintain control and avoid rollovers. The electronic stability control systems, which go by many marketing names such as Electronic Stability Program (ESP) and Vehicle Stability Control (VSC), have been found by Consumers Union, the nonprofit organization that publishes Consumer Reports magazine, to help drivers maintain control. Two years ago, CU advocated the systems be made standard equipment on all SUVs.

But stability systems haven't been endorsed by NHTSA or the Insurance Institute for Highway Safety (IIHS) as an anti-rollover aide. "We're looking right now at electronic stability control as a possible way to reduce rollovers, but we don't have enough data yet," said IIHS spokesman Russ Rader.

In the 2004 model year, Toyota became the first automaker to incorporate stability control on all its mainstream SUVs.

The IIHS' Rader added that new SUV designs also are helping to reduce rollover tendencies. Another feature that can help reduce rollovers is variable ride height suspension systems that lower a vehicle's height when it's traveling at high speeds. Such a system is standard on Land Rover's Ranger Rover, for example.

Lastly, new seat belt designs may help reduce rollover fatalities by better keeping occupants inside their vehicles, where they have a higher survivor rate than if they are ejected.

Ann Job is a writer for T&A Ink media group.