Description of NHTSA Rollover Resistance Ratings

Most rollover crashes occur when a vehicle runs off the road and is tripped by a ditch, curb, soft soil, or other object causing it to rollover. These crashes are usually caused by driver behavior such as speeding or inattention. These are called single vehicle crashes because the crash did not involve a collision with another vehicle. More than 10,000 people die each year in all rollover crashes.

The Rollover Resistance Rating is an estimate of your risk of rolling over if you have a single vehicle crash. It does not predict the likelihood of that crash. The Rollover Resistance Rating essentially measures vehicle characteristics of center of gravity and track width to determine how "top-heavy" a vehicle is. The more "top-heavy" the vehicle, the more likely it is to roll over. The lowest rated vehicles (1-star) are at least four times more likely to roll over than the highest rated vehicles (5-stars).

The Rollover Resistance Ratings of vehicles were compared to 220,000 actual single vehicle crashes, and the ratings were found to relate very closely to the real-world rollover experience of vehicles.

While the Rollover Resistance Rating does not directly predict the risk of injury or death, keep in mind that rollovers have a higher fatality rate than other kinds of crashes. Remember: Even the highest rated vehicle can rollover, but you can reduce your chance of being killed in a rollover by about 75% just by wearing your seat belt.

Here are the Rollover Resistance Ratings:

In A Single Vehicle Crash, a vehicle with a rating of:

Five Stars - Has a risk of rollover of less than 10 percent Four Stars - Has a risk of rollover between 10 percent and 20 percent Three Stars - Has a risk of rollover between 20 percent and 30 percent Two Stars - Has a risk of rollover between 30 percent and 40 percent One Star - Has a risk of rollover greater than 40 percent

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FREQUENTLY ASKED QUESTIONS ABOUT ROLLOVER RESISTANCE RATINGS

Why is consumer information for vehicle rollovers necessary?

While rollovers do not occur as frequently as other types of crashes (see Figure 1), when they do occur, the result is often serious injury or death. Rollovers accounted for more than 10,000 fatalities in the United States in 1999, more than side and rear crashes combined (see Figure 2). They also resulted in thousands of serious injuries. NHTSA believes that most of these rollovers, and the tragic injuries that result, are preventable, if consumers understand the roles the driver, roadside environment and vehicle play in causing the rollover.

How do most vehicle rollovers occur?

From its studies of real-world single-vehicle crashes, NHTSA has determined that more than 90% of rollovers occur after a driver runs off of the road (see Figure 3). This does not refer to vehicles trying to negotiate difficult trails away from public roads. It refers to vehicles rolling over off of the pavement after the driver has lost control of the vehicle. Once the vehicle slides off of the pavement, a ditch, soft soil, curb or other tripping mechanism usually initiates the rollover.

How should the consumer interpret NHTSA'S Rollover Resistance Ratings?

The Rollover Resistance Ratings are based on Static Stability Factor, which is essentially a measure of how top heavy a vehicle is. The Rollover Resistance Ratings of vehicles were compared to 220,000 actual single vehicle crashes, and the ratings were found to relate very closely to the real-world rollover experience of vehicles. Based on these studies, NHTSA found that taller, narrower vehicles, such as sport utility vehicles (SUVs), are more likely than lower, wider vehicles, such as passenger cars, to trip and roll over once they leave the roadway. Accordingly, NHTSA awards more stars to wider and/or lower vehicles. The Rollover Resistance Rating, however, does not address the causes of the driver losing control and the vehicle leaving the roadway in the first place.

Does a vehicle with a higher Rollover Resistance Rating mean it is immune from rollovers?

No, even a five-star vehicle has up to a 10 percent risk of rolling over in a single vehicle crash. In fact, because of the aggressive way in which the vehicle is driven and/or the age and skill of the driver, certain five-star vehicles such as sports cars, may have a higher number of rollovers per hundred registered vehicles than certain three-star vehicles, such as minivans, due to the fact that they are in more single vehicle crashes.

How does Electronic Stability Control affect rollover, and what is its relationship to the Rollover Resistance Ratings?

Most rollovers occur when a vehicle runs off the road and strikes a curb, soft shoulder, guard rail or other object that "trips" it. The Rollover Resistance Ratings estimate the risk of rollover in event of a single vehicle crash, usually when the vehicle runs off the road. Electronic Stability Control (which is offered under various trade names) is designed to assist drivers in maintaining control of their vehicles during extreme steering maneuvers. It senses when a vehicle is starting to spin out (oversteer) or plow out (understeer), and it turns the vehicle to the appropriate heading by automatically applying the brake at one or more wheels. Some systems also automatically slow the vehicle with further brake and throttle intervention. What makes Electronic Stability Control (ESC) promising is the possibility that with its aid many drivers will avoid running off the road and having a single vehicle crash in the first place. However, ESC cannot keep a vehicle on the road if its speed is simply too great for the available traction and the maneuver the driver is attempting, or if road departure is a result of driver inattention. In these cases, a single vehicle crash will happen, and the rollover resistance rating will apply as it does to all vehicles in the event of a single vehicle crash. Some of the 2001 model year vehicles that will be rated have ESC and are identified in the charts with the Rollover Resistance Ratings.

What other information does a consumer need to know in order to minimize the chances of rollover?

A rollover crash is a complex event, heavily influenced by driver and road characteristics, as well as the design of the vehicle. Consequently, a consumer should also know that:

All Vehicles Can Roll Over

All types of vehicles roll over in certain conditions. While SUVs have the highest number of rollovers per 100 crashes (see Figure 4), because of the higher numbers of passenger cars on the road, almost half of all rollovers which occurred in 1999 involved passenger cars (see Figure 5).

Rollovers Are More Likely on Rural Roads and Highways

When a vehicle goes off rural roads it is likely to overturn when it strikes a ditch or embankment or is tripped by soft soil (see Figure 6). Many other rollover crashes occur along freeways with grassy or dirt medians when a driver loses control at highway speeds and the vehicle slides sideways off the road and overturns when the tires dig into the dirt (see Figure 7).

What can the consumer do to reduce rollover risk?

Since most vehicle rollovers are single-vehicle crashes, they are often preventable. They are unlike non-rollover multiple-vehicle crashes involving frontal, side and rear impacts, where another driver may have been responsible for the crash. To minimize the risk of a rollover crash and serious injury, the driver should:

Always Wear Seat Belts Regardless of vehicle choice, the consumer and his or her passengers can dramatically reduce their risk of being killed or seriously injured in a rollover crash by simply using their seat belts. Seat belt use has an even greater effect on reducing the deadliness of rollover crashes than on other crashes because so many victims of rollover crashes die as a result of being partially or completely thrown from the vehicle. NHTSA estimates that belted occupants are about 75% less likely to be killed in a rollover crash than unbelted occupants.

Avoid Conditions That Lead To Loss Of Control Common reasons drivers lose control of their vehicles and run off of the road include: driving under the influence of alcohol or drugs, driving while sleepy or inattentive, or driving too fast for the conditions.

Be Careful on Rural Roads Drivers should be particularly cautious on curved rural roads and maintain a safe speed to avoid running off the road and striking a ditch or embankment and rolling over.

Avoid Extreme Panic-like Steering Another condition which may cause a rollover is where a driver overcorrects the steering as a panic reaction to an emergency or to something as simple as dropping a wheel off the pavement (See Figure 8). Especially at freeway speeds, over correcting or excessive steering may cause the driver to lose control resulting in the vehicle sliding sideways and rolling over. If your vehicle should go off the roadway, gradually reduce the vehicle speed and then ease the vehicle back on to the roadway when it is safe to do so (See Figure 9).

Maintain Tires Properly Since maintaining vehicle control is the most important factor in minimizing the chances of a vehicle rollover, improperly inflated and worn tires can be dangerous. Worn tires may cause the vehicle to slide sideways on wet or slippery pavement, resulting in the vehicle sliding off the road and increasing the risk of rolling over. Improper inflation can accelerate tire wear, and can even lead to catastrophic failures. It is important that consumers maintain tires properly and replace them, when necessary.

Load Vehicles Properly Consult your owner's manual to determine the maximum safe load for your vehicle, and the proper distribution of that load. Pay special attention to the vehicle manufacturer's instructions and weight limits when using any type of roof rack. Any load placed on the roof will be above the center of gravity of the vehicle and will increase the likelihood of rolling over.