Vehicle Rollovers and How to Prevent Them

Rollovers are dangerous incidents that have a higher fatality rate than other types of crashes. Each year over 10,000 people lose their lives in rollovers and they account for 33% of all passenger vehicle fatalities. While narrow and tall vehicles are more susceptible to roll overs all vehicles can rollover. You are best to observe the speed limits and avoid drinking and driving to prevent a rollover. And please wear a seat belt, you are 75% more likely to survive a rollover accident. Here are some more tips on rollover characteristics, how to avoid rollovers, and new safety equipment that also helps.

First the bad news….

Rollovers are dangerous incidents and have a higher fatality rate than other kinds of crashes. Of the nearly 9.1 million passenger car, SUV, pickup and van crashes in 2010, only 2.1% involved a rollover.

However, rollovers accounted for nearly 35% of all deaths from passenger vehicle crashes. In 2010 alone, more than 7,600 people died in rollover crashes. The majority of them (69%) were not wearing safety belts.

So what kinds of rollovers are there?

TRIPPED ROLLOVERS: NHTSA data show that 95% of single-vehicle rollovers are tripped . This happens when a vehicle leaves the roadway and slides sideways, digging its tires into soft soil or striking an object such as a curb or guardrail. The high tripping force applied to the tires in these situations can cause the vehicle to roll over.

Soft Soil: The driver of a pickup truck traveling down a 2-lane highway at a high rate of speed veers off the right side of the road, attempts to recover by steering too sharply to the left and causes the vehicle to roll over as its wheels dig into the soft soil. Curbs, soft soil/shoulders, guardrails, pavement surface discontinuities, snow banks, or other objects can cause tripping.

Guard Rail: The driver of a passenger car traveling down a 2-lane highway at a highlight rate of speed veers off the right side of the road, attempts to avoid the guardrail by steering to the right. The left front portion of the car rides up the guardrail in a ramp-like fashion, causing the vehicle to become airborne and rollover. Tripping can also occur when a vehicle is traveling forward, typically at a high speed. If one side of the vehicle rides up on an object, like a guardrail, it may be forced to roll over.

Steep Slope: A sports utility vehicle traveling very slowly in an off-road environment begins to pick up speed as it rides down a steep hill. The driver misjudges the steepness of the hill as a turn to the right is attempted. The vehicle rolls over and tumbles down the hill. Tripping can also occur on severe slopes in off-road situations. If an incline’s slope is too steep to keep the vehicle upright, it can topple over.

UN-TRIPPED: Un-tripped rollovers are less common than tripped rollovers, occurring less than 5% of the time, and mostly to top-heavy vehicles. Instead of an object serving as a tripping mechanism, un-tripped rollovers usually occur during high-speed collision avoidance maneuvers.

Collision Avoidance: The driver of a heavily loaded pickup truck traveling down a 2-lane highway at a high rate of speed swerves to the left to avoid two wooden crates in the right-hand lane. The driver attempts to recover by turning abruptly to the right, however the top heaviness of the pick up truck with a full load in the bed causes the vehicle to rollover, tumbling down the roadway

Here are some things that cause rollovers…

Rollovers are complex crash incidents and are particularly violent in nature. Rollovers, more so than other types of crashes, reflect the interaction of the driver, road, vehicle, and environmental factors. So while vehicle type does play a significant role, other factors such as driver behavior and road and environmental conditions can also cause a vehicle to roll over.

VEHICLE TYPE: All types of vehicles can rollover. However, taller, narrower vehicles such as SUVs, pickups, and vans have higher centers of gravity, and thus are more susceptible to rollover if involved in a single-vehicle crash.

SPEED: Fatal rollover crashes are speed-related more often than fatal non-rollover crashes. Some 40% of fatal rollover crashes involved excessive speeding . Additionally, nearly ¾ of fatal rollovers took place where the posted speed limit was 55 miles per hour or higher.

ALCOHOL: Nearly half of all fatal rollover crashes involve alcohol . Impairment can result from any blood alcohol concentration (BAC) above .00 . Even a small amount of alcohol will negatively affect your judgment, muscular coordination, and vision, making you more likely to lose control of your vehicle.

LOCATION: Rural roads tend to be undivided and without barriers. They are thus more likely to be the scene of a fatal rollover. Almost ¾ of fatal rollovers occur in rural areas where the posted speed limit is typically 55 miles per hour or higher.

ROUTINE DRIVING: NHTSA data also suggest that over 90% of the vehicles in fatal, single-vehicle rollover crashes were involved in routine driving maneuvers (going straight or negotiating a curve) at the time of the crash. This further suggests that driver behavior (distraction, inattentiveness, speeding, and impaired driving) plays a significant role in rollover crashes.

SINGLE-VEHICLE CRASHES: NHTSA data show that nearly 85% of all rollover-related fatalities are the result of single-vehicle crashes. This means that the majority of rollover crashes and fatalities do not involve any other vehicle besides the one that rolled over, further suggesting that driver behavior plays a significant role in rollover crashes.

Safety features on newer cars to help prevent rollovers…

Electronic Stability Control:  Available in many new cars, this technology helps drivers maintain control of their vehicle during extreme steering maneuvers by keeping the vehicle headed in the driver’s intended direction, even when the vehicle nears or exceeds the limits of road traction.

When drivers attempt an extreme maneuver (for example, to avoid a crash or because a curve’s severity has been misjudged), they may experience unfamiliar vehicle handling characteristics as the vehicle nears the limits of road traction. The result is a loss of control. This loss usually results in either the rear of the vehicle “spinning out,” or the front of the vehicle “plowing out .”

A professional driver, with sufficient road traction, could maintain control in an extreme maneuver by using various techniques, such as counter steering (momentarily turning away from the intended direction). It would be unlikely, however, for an average driver to properly apply counter steering techniques in a panic situation to regain vehicle control.

Electronic Stability Control (ESC) uses automatic braking of individual wheels to prevent the heading from changing too quickly (spinning out) or not quickly enough (plowing out). ESC cannot increase the available traction, but maximizes the possibility of keeping the vehicle under control and on the road during extreme maneuvers by using the driver’s natural reaction of steering in the intended direction.

ESC happens so quickly that drivers do not perceive the need for steering corrections. If drivers do brake because the curve is more or less sharp than anticipated, the system is still capable of generating uneven braking if necessary to correct the heading.

ESC systems exist under many trade names, including Vehicle Stability Control (VSC), Electronic Stability Program (ESP), and Vehicle Stability Enhancement (VSE).

Rollover Air Bags: In addition to protecting drivers’ or passengers’ heads during a side-impact crash, some side-impact head air bags, or “curtains,” can also protect occupants from injury and ejection during a rollover crash. This is important because ejection causes most injuries and fatalities in rollover crashes – most people who are killed are not wearing safety belts to hold them in place.  Not all side-impact head air bags are designed to deploy as rollover air bags. Check with your dealer and vehicle manufacturer for the availability of side-impact head air bags that can also operate as rollover air bags.

If a rollover is detected, the side-impact head air bags are typically triggered in combination with safety belt retractors to remove slack from the safety belt and keep the occupant firmly in the seat. Most side-impact head air bags deploy downward from the overhead roof rail, very close to the side windows. In many cases the rollover sensing system can determine an imminent rollover when the roll angle is very small and all four wheels are still on the ground.

When deployed as rollover air bags, side-impact head air bags will stay inflated longer to help protect the heads of the occupants during the rollover. They also keep the occupants of the outboard seats from being thrown from the vehicle. The combination of these air bags and properly worn safety belts can significantly reduce the chance of ejection.

Variable Ride-Height: Depending on conditions such as vehicle speed and terrain, Variable Ride-Height Suspension (VRHS) raises or lowers the ride height of the vehicle while it is in motion. Some VRHS systems operate automatically, while others require the driver to select the appropriate mode. VRHS systems can have a favorable effect on a vehicle’s likelihood to roll over because they lower the height of the vehicle’s center of gravity and improve its Static Stability Factor (SSF) rating for highway driving.   VRHS systems exist under many trade names, including Electronic Height Control (EHC), Active Height Control (AHC), and pneumatic suspension systems.

How VHRS works: In general, during off-road, lower-speed conditions, the vehicle will ride higher to allow for increased ground clearance. During on-road, higher speed conditions, the vehicle will ride lower. The vehicle riding lower by several inches is less top-heavy, has a higher SSF, and is less likely to roll over in a crash.

What is a Static Stability Factor (SSF) and how is it computed? The Static Stability Factor (SSF) of a vehicle is an at-rest calculation of its rollover resistance based on its most important geometric properties. SSF is a measure of how top-heavy a vehicle is.

A vehicle’s SSF is calculated using the formula SSF=T/2H, where T is the “track width” of the vehicle and H is the “height of the center of gravity” of the vehicle. The track width is the distance between the centers of the right and left tires along the axle. The location of the center of gravity is measured in a laboratory to determine the height above the ground of the vehicle’s mass. The lower the SSF number, the more likely the vehicle is to roll over in a single-vehicle crash.

What can I do to help prevent rollovers?

Avoid Panic-like Steering: Many rollovers occur when drivers over correct their steering as a panic reaction to an emergency—or even to a wheel going off the pavement’s edge. At highway speeds, over correcting or excessive steering can cause the driver to lose control, which can force the vehicle to slide sideways and roll over.

Know Proper Maneuvering: If your vehicle leaves the roadway, gradually reduce speed. Then, when it’s safe to do so, ease the vehicle back onto the roadway.

Tire Pressure and Loading Limits: 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 goes off a rural road, the vehicle can overturn when it strikes a ditch or embankment, or is tripped by soft soil. Nearly 75% of all rollover crashes occur in rural areas, so practice caution when driving on rural roads.

Maintain Your Tires: Improperly inflated and worn tires can be especially dangerous because they inhibit your ability to maintain vehicle control, the most important factor in reducing the chance of rollover. Worn tires may cause the vehicle to slide sideways on wet or slippery pavement, sliding the vehicle off the road and increasing its risk of 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.

Load Vehicles Properly: 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. Any load placed on the roof will be above the vehicle’s center of gravity, and will increase the vehicle’s likelihood of rolling over.

Use Caution 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 goes off a rural road, the vehicle can overturn when it strikes a ditch or embankment, or is tripped by soft soil. Nearly 75% of all rollover crashes occur in rural areas, so practice caution when driving on rural roads.

DISCLAIMER AND LIMITATION OF LIABILITY: This information is provided as a public service and information was gleaned from the National Highway Traffic Safety Administration (NHTSA), U.S. Department of Transportation (DOT). The NHTSA/DOT makes no claims, promises or guarantees about the accuracy, completeness, or adequacy of the contents of this article and expressly disclaims liability for errors and omissions in the contents of this information.  No warranty of any kind, implied, expressed or statutory, including but not limited to the warranties of non-infringement of third party rights, title, merchantability, fitness for a particular purpose and freedom from computer virus, is given with respect to the contents of this web site or its links to other Internet resources. Reference in this web site to any specific commercial products, processes, or services, or the use of any trade, firm or corporation name is for the information and convenience of the public, and does not constitute endorsement, recommendation, or favoring by NHTSA/DOT.


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