- How common are rollovers? How serious are they?
- When are rollovers most likely to occur?
- Do rollovers occur with all vehicles?
- I've heard that 15 passenger vans roll over easily. Is this true?
- What are the National Highway Traffic Safety Administration's Vehicle Rollover Ratings?
- What can I do to reduce my risk of being involved in a rollover?
- What can I do to protect myself if my vehicle does roll over?
- What is the Static Stability Factor? What is a good Static Stability Factor?
Rollovers came into the public spotlight in 2001, when the Ford Motor Company recalled many Ford Explorers that used Bridgestone/Firestone tires. In many high-profile cases, the tires in question failed during high speed driving and contributed to rollovers. In 2003, the National Highway Traffic Safety Administration (NHTSA) declared rollover safety a priority issue, and in 2004, they released a report on 15-passenger vans, describing their increased risk of rolling over when heavily loaded. As the composition of the passenger vehicle fleet has shifted over the past couple of decades to include more SUVs, pickup trucks, and vans, rollovers have gained increasing attention; however, rollover is not by any means a new phenomenon. Vehicles have been rolling over virtually ever since people have been driving, and the AAA Foundation funded research on rollover-related injuries as long ago as 1991, when we published a report entitled, “The Contribution of Rollover to Single-Vehicle Crash Injuries.” Published studies of rollovers date back as far as the 1950s.
How common are rollovers? How serious are they?
Rollovers are relatively rare crashes; however, when they occur, they are much more likely than most other types of crashes to result in serious injury or death. Even though only about one of every forty vehicles involved in a police reported crash in recent years has rolled over, one of every three passenger vehicle occupant deaths occur in rollovers, illustrating the relative seriousness of rollover crashes.
When are rollovers most likely to occur?
According to NHTSA, the overwhelming majority of rollovers occur duringordinary driving situations. Many rollovers occur when a driver suddenly swerves to avoid an obstacle such as a stopped car in the road or when a driver accidentally veers off of the road. In fatal rollovers, excessive speed and alcohol are often contributing factors. Nearly 3 of every 4 fatal rollovers occurs on a rural road with a posted speed limit of 55 mph or higher, and excessive speeding is cited as a contributory factor in about 40% of fatal rollovers. About half of all fatal rollovers involve alcohol in at least some capacity (i.e., drivers with BAC > 0.00, though not necessarily in excess of the legal limit for DWI). More than 80% of all rollovers involve no other vehicle besides the one that rolls over.
Do rollovers occur with all vehicles?
Yes. Although both basic physics and real world crash statistics suggest that some vehicles are more likely than others to roll over, any vehicle can roll over. According to statistics from the National Highway Traffic Safety Administration, half of rollovers occurring between 1995 and 2001 involved passenger cars, and half involved light trucks (i.e., SUVs, pickup trucks, etc.) or vans. However, the passenger vehicle fleet comprised roughly 65% passenger cars and 35% light trucks and vans during the period of the study, indicating that a higher proportion of all light trucks and vans on the road are involved in rollovers.
This increased susceptibility to rolling over is due largely to the relatively high height of the center of gravity of these vehicles, as well as the propensity of some of these vehicles to oversteer (i.e., the rear of the vehicle sliding “outward” when turning, swerving, or negotiating a curve). For a given track width, a vehicle with a higher center of gravity will tip more easily; however, even very low-to-the-ground sports cars can (and do) roll over.
I've heard that 15 passenger vans roll over easily. Is this true?
When lightly loaded, 15-passenger vans are quite similar to other light trucks in terms of their handling characteristics. However, 15-passenger vans can hold many more people than SUVs, pickup trucks, or minivans can hold. When more passengers climb in, the vehicle's center of gravity moves higher and further rearward, which both increases the vehicle's propensity to oversteer and decreases its lateral stability. As a consequence, the rollover risk of a heavily loaded 15-passenger van is increased, especially during emergency maneuvering or high speed driving. The National Highway Traffic Safety Administration's Consumer Advisory on 15-passenger vans issues several recommendations for reducing rollover risk, and this technical report describes the physical basis for the risk of rollover in these vehicles. Some of the key recommendations from NHTSA are:
- Operators of 15-passenger vans should be highly experienced in driving these vans (especially when transporting several passengers).
- When not all seating positions are occupied, seats forward of the rear axle should be occupied first (i.e., don't have everybody sit in back).
- Drive extra carefully: reduce speed in bad weather, avoid distractions, and do not drive when tired.
- Do not transport cargo on the roof, this further raises the center of gravity and increases risk of rolling over.
- Everybody buckle up! When rollovers occur, the people most often seriously injured or killed are the ones who aren't wearing their seatbelts.
What are the National Highway Traffic Safety Administration's Vehicle Rollover Ratings ?
Starting with 2004 model year vehicles, rollover ratings are labeled as the combined rollover resistance rating, derived from a geometric calculation called the Static Stability Factor (SSF), and the results of a dynamic maneuvering test that simulates a high-speed collision avoidance maneuver. These results are then combined for one overall star rating. Model year 2003 and earlier vehicles were also assigned star ratings; however, these ratings were assigned according to SSF alone, and thus are not strictly comparable to ratings of the 2004 model year vehicles.
What can I do to reduce my risk of being involved in a rollover?
Keep your vehicle in good condition and drive carefully. According to the National Highway Traffic Safety Administration, four of every five rollovers involve no other vehicle besides the one that rolls over, so in this regard, you as a driver are largely in control. Here are some important and simple steps to take to keep yourself safe:
Don't Drive Too Fast - The posted speed limit is an upper limit, not a lower limit. The faster you drive, the less time you have to react to any emergency that suddenly arises in the road ahead of you, which means you will probably end up steering more sharply and/or braking harder, both of which compromise your ability to safely control your vehicle.
Steering - Many rollovers occur when drivers overcorrect their steering in response to unexpected situations, such as encountering a stopped vehicle in their lane or accidentally driving off of the pavement. Sudden steering maneuvers at high speeds or on soft surfaces can lead to rollovers.
Know proper maneuvering - If your vehicle leaves the paved road surface, slow down gradually , don't stomp on the brakes. Then, when it's safe to do so, ease the vehicle back onto the roadway. Don't suddenly jerk the steering wheel to get the vehicle back on the pavement.
Be extra careful on rural roads - Rollovers are more likely to occur on rural roads and highways, particularly undivided, two-way roads or divided roads with no barriers. When a vehicle leaves the pavement, it can be tripped by roadside objects or soft surfaces or it can roll down a slope. Nearly 75 percent of all rollover crashes occur in rural areas, so be extra careful when driving on rural roads.
Tires - Improperly inflated and/or worn tires can be especially dangerous, because they inhibit your ability to maintain vehicle control. Worn tires increase the likelihood that the vehicle may slide sideways on wet or slippery pavement, increasing its risk of sliding off of the pavement and rolling over. Improper inflation can accelerate tire wear, and can even lead to tire failure. It is important to maintain your tires properly, and replace them when necessary. Monitor your tire pressure regularly, keeping it within the manufacturer-recommended range for the tire and the vehicle.
Vehicle loading - Consult your vehicle's owner's manual to determine the maximum safe load for your vehicle, as well as proper load distribution. If you're using a roof rack, pay special attention to the manufacturer's instructions and weight limits. Realize that any load placed on the roof will raise the vehicle's center of gravity, increasing the vehicle's likelihood of rolling over.
Select your vehicle carefully - Think carefully about what type of vehicle you want to buy, and why. Will you be driving off road a lot? Will you be hauling cargo? Will you be carrying a large number of passengers? Have you ever driven a truck or van before? If you didn't answer “yes” to any of these, an ordinary passenger car will probably be a sensible choice, and most ordinary passenger cars are considerably less likely to roll over than most pickup trucks, vans, or SUVs during the same driving manuevers. If you find yourself driving a van, pickup truck, or SUV for the first time, it is extremely important to realize that its ride and handling characteristics are different from those of ordinary passenger cars, and driving one of these vehicles requires extreme care, especially if you are unaccustomed to it.
Also, consider buying a vehicle with Electronic Stability Control (ESC). Studies from the Insurance Institute for Highway Safety have found ESC to dramatically reduce the involvement of equipped vehicles in single vehicle crashes involving injury or death. ESC is designed to keep the vehicle “on track,” preventing it from understeering (i.e, the front tires lose traction and the vehicle turns less than you intend) or oversteering (i.e., the rear tires lose traction and the vehicle turns further than you intend). Although study was not able to determine conclusively how ESC impacted rollover risk, the results suggested that it might be beneficial in this regard.
If you are concerned about the rollover risk of the car that you own, or would like to consider rollover risk when buying a new car, consider the National Highway Traffic Safety Administration's Rollover Ratings, available at http://www.safercar.gov/Safety+Ratings
What can I do to protect myself if my vehicle does roll over?
You can substantially reduce your risk of injury by wearing your seatbelt. According to data from the National Highway Traffic Safety Administration, about three quarters of people killed in rollovers were not wearing their seatbelts, and almost two thirds of them were thrown out of their vehicles during the rollovers. Although some of the very worst rollovers may be virtually unsurvivable, buckling up will almost always improve your chances of surviving and decrease the severity of any injuries that you're likely to sustain.
What is the Static Stability Factor? What is a good Static Stability Factor?
The SSF is a number that relates the height of the center of gravity of a vehicle to its width, as shown in the figure, below.
Due to more complex issues such as the design of the vehicle's suspension and whether or not the vehicle is equipped with Electronic Stability Control, the SSF alone cannot be used to predict any absolute performance capabilities of a vehicle; however, it is commonly used to compare the relative stabilities of different vehicles. During driving maneuvers that might lead some vehicles to roll over (e.g., sudden swerving or high speed cornering), a vehicle with a lower SSF is generally considered to be more likely to roll over.
Last revised 2/12/2015
Article about Firestone/Bridgestone tire failures (CBS News)
National Highway Traffic Safety Administration (NHTSA) Rollover Information
Traffic Safety Facts 2002 (NHTSA)
Research Note (NHTSA)
Characteristics of Fatal Rollover Crashes (NHTSA)
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