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Rollover 101

How rollovers happen and what you can do to avoid one

Last updated: April 2014

A vehicle rollover is among the worst things that can happen to you on the road. Although rollovers occur in only about 3 percent of all serious crashes, they account for about 30 percent of people killed while riding in a passenger vehicle.  

Rollovers needn't be so deadly. Rollover-avoidance technologies, better vehicle design, enhanced safety systems, stronger government regulations, and increased use of safety belts could cut the number killed and injured by half or more. This primer will explain how rollovers happen, and how the government's rollover tests work.

How rollovers happen. Given the right circumstances, any vehicle can roll over. However, taller, narrower vehicles such as SUVs, pickups, and vans are more susceptible than traditional cars are because they have a higher center of gravity and thus are more top-heavy. Sideways forces that develop when a vehicle rounds a curve shifts the center of gravity to one side, which can have a dramatic effect on the vehicle's balance. The lateral forces increase with speed and also with rapid changes of direction--for example, when a driver makes too sharp a turn one way and then overcorrects the other way. Those transitions can set up a pendulum effect, with larger and larger swings and an eventual loss of control.

A single-vehicle rollover is usually not caused by a steering maneuver. Instead, the vehicle usually has to "trip" on something, such as when it swerves into a curb, pothole, or a soft roadside shoulder. The government has estimated that 95 percent of rollovers result from trips. Some observers say that number is too high. If a vehicle leans in such a way that a tire's sidewall deforms and the wheel rim strikes the pavement and provokes a tip-up, then the government counts that as a tripped rollover.

But Consumer Reports encountered that very phenomenon a few times during its emergency-handling tests of SUVs in the 1980s and 1990s. We consider those tip-ups to be untripped, because the vehicle essentially fell over itself on a flat surface without encountering some obstruction. The distinction is important because the supposed rarity of untripped rollovers has served as an excuse for the government and the auto industry to play down rollover risk and put the blame on the driver and road conditions rather than on the vehicle.

Good grip, but not too much. In any case, tire grip plays a paradoxical role in rollovers. Ideally, your vehicle would stay on course, gripping the road with all four wheels on the ground, no matter what. But too much tire grip can allow excessive sideways forces to build up until the vehicle flips over. Before that happens, you want the vehicle to gradually and predictably lose some lateral grip. Sliding is better than tipping over, but that too can put the vehicle at risk of hitting something during the slide, and then rolling anyway.

One cheap way that automakers make an SUV less prone to rollover is to equip it with less-grippy tires. That can help prevent some rollovers but is obviously a less-than-ideal solution since tire traction keeps you on the road and affects stopping distances.

The tire-grip issue sheds light on another potential source of rollover: those sporty aftermarket SUV and pickup truck tires. If "performance" replacement tires provide more lateral grip than the original-equipment tires, they might increase the chance of rollover in an emergency maneuver. Our advice is to stick with tires that are nearly identical to those that came with the vehicle. The characteristics of those tires formed part of the basis for the vehicle's original safety potential.

Preventing and surviving a rollover

The news is not all bad. When seen in terms of rollover-fatalities per million registered vehicles, all vehicle types have improved, and SUVs have improved the most. According to the IIHS, the rollover driver-death rate among newer (1 to 3 year old) passenger vehicles dropped from 27 in the year 2000 to 6 in 2012. The newest SUVs have lower rates than the newest cars.

In the same period, the number of SUVs on the road increased dramatically, which is why the actual number of deaths hasn't changed much despite the improved survival rates.  

Common sense and routine maintenance can greatly increase your odds of avoiding a rollover and walking away from one if it happens. Here are some survival tips:

Newer is better. The improvement might be because more people are buckling up, or because vehicles have better build quality and safety systems, but it's probably a result of both. Either way, it makes sense to choose a vehicle with the most up-to-date safety systems. Especially important are electronic stability control and side-curtain air bags.

Wear safety belts. Belts help keep you in the seat so you are not tossed around in a rollover crash. About half of rollover fatalities occur when people are partly or completely ejected from the vehicle. Don't think it's good to be "thrown clear." In all kinds of crashes, nearly three-quarters of people ejected from a vehicle are killed.

Check the tires. Make sure all the tires are in good shape and properly inflated to the pressure recommended by the vehicle manufacturer. Check the inflation pressure at least once a month. Replacement tires should be similar to the vehicle's original set.

Watch the load. Overloading any vehicle, particularly SUVs and pickups, decreases its stability. The worst practice is to place heavy loads on the roof. Try to stay well within the load ratings specified by the manufacturer. (They should be noted in your owner’s manual.) Try to place the heaviest cargo low on the floor and as far from the tailgate and as close to the center of the vehicle as possible.

Watch your speed. Speed makes a vehicle's tendencies to roll over more severe, and it also makes more demands on a driver's attention and skill. About 40 percent of fatal rollovers involve excessive speed, the government reports.

Beware on country roads. Almost three-quarters of fatal rollovers occur in rural areas on roads where the speed limit is 55 mph or more. According to the National Highway Traffic Safety Administration, those roads tended to be undivided highways without barriers.

How the government's rollover tests work

The National Highway Traffic Safety Administration (NHTSA) conducts on-road dynamic rollover tests on most new SUVs, pickups, and minivans. The tests score rollover propensity in part by putting vehicles through a handling maneuver called a fishhook, a quick left-right turn, at increasing speeds from 35 to 50 mph. This simulates a driver overcorrecting the vehicle's steering, something that can happen in an emergency.

If the vehicle lifts two wheels off the ground it's considered a "tip-up" and testing stops. If the vehicle slides out or gets through the test without incident at 50 mph, it passes.

Before 2004, NHTSA rated rollover propensity with a static (non-moving) measurement of a vehicle's shape and weight distribution. Called the Static Stability Factor (SSF), it's derived from a formula that compares a vehicle's track width with its center of gravity height. But starting with 2004 models, NHTSA added the dynamic (vehicle in motion) test, and the agency now uses those results to augment its rollover ratings.

NHTSA has compiled rollover ratings for scores of vehicles, including separate ratings for two- and four-wheel drive versions of SUVs. So far, no car or minivan has tipped up. In fact, NHTSA conducts on-road rollover testing on only two sedans per year and assigns star ratings to cars based on their SSF alone, as it did with 2003 and earlier models.

The scores. NHTSA combines the SSF and dynamic test to assign a rollover-resistance score of one to five stars. Five stars represents rollover likelihood in a single-vehicle crash at 10 percent or less; one star predicts a rollover likelihood of 40 percent or more. The SSFs underlying the star ratings vary from about 1.0 to 1.5. (The higher that number, the better.) SUVs usually measure out at 1.0 to 1.3, and cars normally fall in the range of 1.3 to 1.5.

What the results mean. Many SUVs have tipped up in NHTSA's testing—understandable since the dynamic test is designed to provoke a tip-up if a steering maneuver alone can produce one. What is surprising is that a tip-up doesn't affect the star rating much. That's because the government's rollover ratings give much more weight to the static measurement than to the on-road test. NHTSA's rationale for this is that the dynamic test looks for an untripped rollover tendency, which NHTSA believes are rare in real life.

Consumer Reports takes the position that if a vehicle can tip up in a steering maneuver, without impacting anything else first, then that's a good reason to look for a less tip-prone alternative.  

NHTSA's rollover ratings can be found at www.safercar.gov. For specific information about a vehicle's star rating, click on "Search 5-Star Safety Ratings," then select the vehicle class, such as SUV, then its year, then the make and model. Scroll down to the heading Rollover, and a chart there will tell you whether the vehicle tipped (under Dynamic Test Result), and also its likelihood of rollover expressed as an exact percentage rather than a star.

You can also see lists of all tested vehicles within a class (passenger car, SUV, etc.) starting with the www.safercar.gov home page, clicking on "Search 5-Star Safety Ratings" then selecting just the class, or class and model year.

 

Roof rule strengthened but rollover standards still lacking

It might come as a surprise, but the government doesn't have any standards that involve rolling a vehicle over. Instead, it runs a public-education program that rates rollover propensity. But that's not the same as a performance standard, which would require some level of occupant protection when a rollover happens.

In lieu of a rollover standard based on testing a moving vehicle, the government has long had a “roof-crush” rule based on testing vehicles when they are stationary. Roof integrity is certainly important. A collapsing roof can kill or injure people no matter how well they are otherwise restrained. NHTSA has estimated that a collapsing roof kills about 600 and injures about 900 people every year, even though they were belted in.

The roof-crush regulation was finally updated, after many years of deliberation, in 2009. That was the first significant update in 38 years. The mandatory three-year phase-in period starts with vehicles manufactured in September, 2012.  This rule change could represent a significant safety improvement, but some safety advocates say the proposal doesn't go far enough.

Background on rollover-resistance standard.
The old "roof crush" requirement was promulgated in 1973. That regulation is Federal Motor Vehicle Safety Standard 216. It required automakers to subject a sample of every model to a roof-crush test before it could be sold. In that test a stationary vehicle had a weight pressed against one edge of its roof. The roof had to withstand a force equivalent to 1.5 times the weight of the vehicle, up to a limit of 5,000 pounds, without the plate moving more than 5 inches.

Revised roof-crush rules. The newest (2009) rule says that vehicles weighing 6,000 pounds or less must be able to withstand a force equal to three times their weight applied alternately to the left and right sides of the roof. The roof cannot bend so far that it would touch the head of a median-height-male test dummy.

While the heaviest passenger vehicles, those weighing between 6,000 and 10,000 pounds, used to be exempt, they are now covered by the standard. However, those heavier vehicles need only withstand 1.5 times their own weight on the roof.

Joan Claybrook, president of safety watchdog Public Citizen, says the standard fails in three ways:

It does not require the plate pressing on the roof to be angled farther forward to better simulate real rollovers.

It does not apply enough force. Experts agree that to withstand the forces of a real rollover, roofs should support about four times the vehicle weight, not three times their weight as the new rule specifies.

Safety belts are not required to hold occupants in place during a rollover. As cars roll, occupants are pulled out of their seats and toward the roof. Most safety belts in use today won't stop that.

Limits on lawsuits. Perhaps the most worrisome part of the proposal is language that might limit lawsuits against automakers. Under the new rule, injured occupants could not make a legal claim that automakers had any obligation to make roofs stronger than the current standard requires, even where state courts had previously held manufacturers to a stricter standard. That means many cases involving crushed roofs could be dismissed without trial, because the new rule, which includes weak standards, would pre-empt them. The government, however, dismisses that concern, The Executive Summary to the final rule sates, “We do not foresee any potential State tort requirements that might conflict with today's final rule. Without any conflict, there could not be any implied preemption.”

Safety advocates have long argued that gently applying weight to a vehicle’s roof—which is how current “static” rollover testing is done—is not a proper approximation of what happens when a speeding car rolls over, especially one that rolls over several times before coming to rest. The problem, however, has been finding a test that actually throws a car onto its roof with repeatable results.

NHTSA has argued that electronic stability control, adopted universally in model-year 2012 vehicles, will, by preventing rollovers in the first place, save more lives than stronger roofs would. That is true. However, we continue to believe that stronger roofs are still a desirable feature.

In 2009, the Insurance Institute for Highway Safety (IIHS) introduced its own roof-crush test that subjects a vehicle’s roof to four times the vehicle’s weight. Given the high profile of IIHS, the Institute’s test will undoubtedly influence manufacturers to engineer vehicles that meet those requirements.

   

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