UNSTABLE VEHICLES -- UNSAFE AT ANY SPEED
By Raymond P. Johnson, Esq.
Law Offices of Raymond P. Johnson
10990 Wilshire Boulevard
Suite 1150
Los Angeles, California 90024
June 1993
Introduction
Certain vehicles are prone to roll over in accidents. High speeds are unnecessary. Just a minor collision, a swerve to miss a child, or a drift to the shoulder of the road is enough. When the driver corrects, rather than braking, sliding or skidding to a safe stop, the vehicle rolls. The results can be tragic, including death, paralysis or other maiming injury.1
The cause? Overcorrection, mishandling of the vehicle -- no, not in most rollovers that involve what have become the family station wagons of the 1990s: sports utility vehicles, minivans, and four-wheel drive (4WD) light trucks.
To determine the real cause, take one step closer to the analysis. Look at the vehicle just standing there. Its top-heavy design (high center of gravity) and narrow width are telltale signs. Try the acid test: given the way the accident happened, would rollover have occurred in a traditional station wagon or other vehicle with wider wheel base and/or lower center of gravity? If not, the truth surfaces: the basic inherent instability of the vehicle itself. On the highways and roads of America, these unstable vehicles are loaded weapons, waiting for someone or something to trigger disaster.2
Necessary Definitions
Track Width and Center-of-Gravity Height
To look closer yet, we have to define some technical terms. The track width of a vehicle is measured side to side from center of the right tire to center of the left tire. Track width is usually referred to as "T."
The center of gravity is a theoretical yet critical design point in the vehicle's mass where the vehicle can be perfectly balanced in every direction. Engineers routinely make design decisions based on the premise that the entire weight of a vehicle can be thought of as concentrated at its center of gravity.
The height above the ground of a vehicle's center of gravity is normally designated "h." In a top-heavy vehicle, the center of gravity is higher than normal, and the value of "h" is correspondingly greater. The diagram below gives pictorial definitions of "T" and "h."
[insert diagram]
Stability Ratio
Every vehicle has a static "stability ratio" (SR) defined as one-half its track width (T/2) divided by its center-of-gravity height (h), or
SR = T/2h
A higher SR indicates a more stable vehicle. Looking at the formula, one can increase SR or stability by increasing T (making the track width wider) or decreasing h (making the vehicle less top heavy).
Problems That Can Kill and Maim
Static Stability
The value of SR for a particular vehicle is the starting point in determining if the design is inherently unstable. Engineering analyses have shown, study after study, that a vehicle with a static stability ratio (SR) in the vicinity of 1.2 or less is suspect, and a potential danger.3 Tests and accident statistics have demonstrated that even standing still, particular vehicles in this group can be hit in the side or rear at less than 15 miles per hour, and will tip rather than slide out. The vehicle's top-heaviness and narrow width cause it to roll.
Sports utility vehicles, minivans and 4WD light trucks with an SR value of 1.2 or less are prevalent on American highways. For example, certain models of the following vehicles have been reported to fall in that category: Jeep CJ5, Jeep CJ7, Suzuki Samurai, Geo Tracker, Isuzu Trooper and Trooper II, Toyota Land Cruiser, Toyota 4Runner, Mitsubishi Montero, Ford Bronco II, Chevrolet Blazer, GMC Jimmy, International Scout, and 4WD pick-up trucks by Toyota, Nissan and Ford. Others of course exist.
In addition, static stability problems with 4WD light trucks, for example, are further exacerbated by manufacturers who market extended passenger cabs for rear seating, enclosure tops for the back of pick-up trucks, roof racks, oversized tires, and heavy suspension kits. All of these items increase the top-heaviness or center-of-gravity height (h) and decrease the already low stability ratio (SR) of these vehicles.
Crashworthiness
Sports utility vehicles, minivans, and 4WD light trucks are very popular market items in the United States. They are sold as rugged, sure-footed, "go anywhere" vehicles with room for friends and family.4 Yet many models, in addition to having an SR of 1.2 or less, lack crashworthiness. In particular, when these vehicles roll, occupants can be and are seriously injured or killed by crushing roof structures, inadvertent door openings, defective seat belts and/or popping windows that allow occupant ejection.5
Studies dating back to the 1950s show that rollovers cause excessively high rates of serious injury.6 Despite four decades of knowledge, many of these unstable vehicles still are built flimsily, without metal safety cages, crumble zones, or effective roll bars to ensure adequate occupant survival space. Others are built without adequate door strength or window adhesion leading directly to occupant ejections. The result: a continuing litany of highway deaths and injuries that are all too predictable.
Dynamic Stability
In addition, many of these "too high/too narrow" vehicles also suffer from dynamic instability problems. As mentioned, the static stability ratio (SR) is only a starting point in the analysis. SR is a theoretical quantity that assumes a vehicle is perfectly rigid. And, of course, vehicles are not perfectly rigid.
The dynamic factors -- flexible rolling tires, suspension springs and parts, uneven braking, and overly sensitive steering systems -- all mean that side forces (even less than those predicted by the static stability ratio) can cause these vehicles to roll. When these side forces are caused only by friction between the tires and road, a vehicle should simply slide out to a safe stop. With inherently unstable vehicles, however, defective static and dynamic stability features combine instead, and cause the vehicle to roll, with resulting injury and death.
Suspension Systems/Body Lean
For example, an overly active suspension can cause a vehicle's body to rise suddenly, above the chassis and wheels, when coming out of a dip in the road. This, in turn, significantly raises the center-of-gravity height (h) causing the vehicle to react even more unstably to any side forces. In turning maneuvers, that same suspension system can cause the vehicle's upper body to sway or lean off its chassis, further increasing instability and the vehicle's propensity to roll.
Oversteer
Certain types of steering systems make the situation worse. Vehicles that oversteer, for example, are characterized by the rear of the vehicle coming around to the front during tighter turning maneuvers, such as occur when a driver attempts to avoid hitting a child or an animal darting into the street. With oversteer, the rear tires lose traction before the front ones, and the back of the vehicle slides out. As a result, the vehicle ends up sideways.
When oversteer is combined with static instability, a deadly combination results. The tires slide sideways and cannot be steered. The vehicle goes over simply because of frictional side forces on the tires, and the driver and occupants remain helpless throughout.
Sluggish Steering
Dynamic instability is caused further by sluggish or mushy steering. Technically, this condition is known as "delayed yaw response time." It is the time that a vehicle needs because of its design to reach 90% of its final turning rate (yaw) after a steering input is made.
This delay in the vehicle's response to steering input literally can prevent a driver from regaining control after a hard turning maneuver. Because the vehicle takes so long to respond, the driver turns the steering wheel more and more. When the vehicle finally reacts, the driver discovers that a much sharper than intended corrective turn was made. The vehicle goes broadside, side forces from the road build up on the tires, and again unstable vehicles roll, rather than simply and safely sliding out.
Alleged Defenses
The Driver
A favorite defense is "driver error." This strategy, however, actually boils down to "blame the victim," and usually does not play well with juries, especially once the inherently defective nature of these unstable vehicles is understood. An exception: illegal levels of alcohol use.
The jury's propensity to blame the intoxicated driver, however, is really just a by-product of strong public prejudice against drunk drivers. And, although that prejudice can be warranted in many cases, it has little or nothing to do with the defective nature of inherently unstable vehicles, or strict liability for those products.
When unstable vehicles turn sideways, static and dynamic instability, not driver input, cause them to roll. Whether the driver was intoxicated, or instead sober and gifted with the abilities of Mario Andretti is irrelevant. The roll and crash occur because the vehicle tips over as a result of its narrow track width, top-heaviness, and other defects.
In most cases, once the side forces between the tires and road are developed, the driver, sober or not, simply cannot stop the rollover.7 As such, because strict liability focuses on the product, not conduct, motions to exclude evidence of alcohol use (as far more prejudicial than probative) are many times successful under California Code of Evidence § 352 and Federal Rule of Evidence 403.
The Tripping Mechanism
Another well-worn defense is known as the "tripping mechanism." Some manufacturers point to anything -- curbs, rocks, bushes -- and contend that the vehicle rolled because it tripped. The argument: any vehicle, even a sports car, would have rolled in similar conditions.
This alleged defense, however, is usually overcome by careful inspection of the crash scene or related photographs. Skid marks on the road, dirt tracks off the road, or even markings on the vehicle itself usually show that the vehicle was already out of control and at least tipping on two wheels before impact with any probable tripping mechanism. In addition, eye witnesses can be devastating to this purported defense, proving that the vehicle was tipping out of control before any meaningful impact with curbs, rocks, bushes or the like.
High Speed and Violence
Another often-heard defense: speed kills. There is great incentive for the defendant's accident reconstruction expert to make assumptions about such things as the friction surface of the road, the travel path of the vehicle, and other "physical" quantities that drive up speed calculations. Careful coordination, however, with attorneys that specialize in automotive product-liability cases and qualified technical experts will uncover these tactics.8 For example, speeds calculated by the defense expert may be much higher than those evidenced by actual physical proof, such as the indentations and deformation pattern on the post-crash vehicle itself.
In a similar manner, some defense experts try to establish a high-speed roll rate to emphasize the "violence" of the rollover. This obviously helps defendant manufacturers argue that the victim's injury could not have been prevented "without building a tank." Most vehicle rollovers, however, are relatively low energy events; the vehicle dissipates its energy over large distances, as opposed to hitting a wall and grinding to a stop in milliseconds.
Most of the impacts in a typical rollover are glancing blows, decreasing the energy transfer and the corresponding "violence" of each impact. Look at the vehicle itself. Usually, rollover vehicles are characterized by many low-energy impact dents in the areas of the roof, side and hood. This is because the vehicle's motion during the rollover is much more like the glancing blows of a football skimming the ground, than a ping pong ball hitting a wall.
The best evidence of this "footballing" motion is the relatively small amount of deformation or damage caused by the first (highest speed/highest energy) impact between the vehicle and the ground. Again, plaintiffs should emphasize that in determining speed and forces, the physical evidence on the vehicle itself and related photographs are almost always superior to any assumptions about entry speed, vehicle path, coefficients of friction, or similar quantities.
Federal Motor Vehicle Safety Standards
If all else fails, some manufacturers of unstable vehicles will argue that the applicable federal safety standards were met or exceeded. In the case of sports utility vehicles, minivans or 4WD light trucks, however, this argument rings hollow.
The requirements of the Federal Motor Vehicle Safety Standards (FMVSS) pertinent to rollover safety, e.g., roof crush and side impact safety requirements, do not apply to these vehicles. Design discretion is left to the manufacturer. The few government standards that are applicable to sports utility vehicles, minivans and 4WD light trucks are irrelevant to rollover safety, such as hubcap and wheel nut requirements. In addition, the FMVSS do not even address stability or dynamic rollover testing; again, this testing is left to the discretion of the manufacturers.9
Of course, a most important point: the FMVSS, by congressional mandate, are only bare bones or minimal safety requirements for vehicle design.10 Manufacturers have an obligation and duty to build vehicles, without inherent defects, that meet ordinary consumer expectations.11
Conclusion
Many versions of the new family station wagons -- sports utility vehicles, minivans and 4WD light trucks -- are simply too high and too narrow to be safe. Their designs are inherently unstable.
When this basic geometric instability is combined with dynamic stability problems such as oversteer, body lean, and sluggish steering, the results can be disastrous. Just the simple frictional side forces between the vehicle's tires and the road are enough to cause rollover and serious injury.
Attempted defenses alleging driver error, supposed tripping mechanisms and high speeds usually pale in light of the facts. In most cases, these unstable vehicles roll because of their inherent design flaws; the drivers are helpless in their attempts to avoid disaster. The vehicles are simply unstable -- as a matter of design -- and unsafe at any speed.
| 1 |
See, e.g., K. W. Terhune, The Contribution of Rollover to Single-Vehicle Crash Injuries, Calspan Report Number 7881-1, Calspan Corporation, at 46 (1991) (prepared for the AAA Foundation for Traffic Safety under Agreement Number 905) (vehicle rollover increases the risk of serious injury up to 50%).
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| 2 |
See e.g., R. G. Snyder, T. L. McDole, W. M. Ladd, & D. J. Minahan, An Overview of the On-Road Crash Experience of Utility Vehicles, University of Michigan/Highway Safety Research Institute Report Number UM-HSRI-80-15, at 2-4 (1980). (The rate of deaths and disabling injuries are approximately twice as high in utility vehicles. Statistics show that utility vehicles are more than five times more likely to roll than passenger cars.)
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| 3 |
See, e.g., Snyder, supra, at 3 (1980); S. R. Smith, Analysis of Fatal Rollover Accidents in Utility Vehicles, National Highway Traffic Safety Administration (NHTSA) Technical Report DOT HS-806-357, at 23 (1982); D. W. Reinfurt, J. C. Stutts & E. G. Hamilton, A Further Look at Utility Vehicle Rollovers, University of North Carolina/Highway Safety Research Center Report, at 5 (1984); and L. S. Robertson & A. B. Kelley, The Role of Stability in Rollover-Initiated Fatal Motor-Vehicle Crashes Under On-Road Driving Conditions, Automotive Information Exchange Group Report, Birmingham, Alabama, at 2 (1986).
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| 4 |
See, e.g., television advertisements for the Isuzu Trooper II describing these vehicles as "tough," "reliable," "go anywhere," "be anything," "station wagons."
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| 5 |
Terhune, supra, at 39 (ejection results in serious driver injury in 50-80% of all vehicle rollovers).
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| 6 |
See, e.g., J. O. Moore & B. Tourin, A Study of Automobile Doors Opening Under Crash Conditions, Automotive Crash Injury Research Report, Cornell University Medical College (1954); and J. W. Garrett, A Study of Rollover in Rural United States Automobile Accidents, 12th Stapp Car Crash Conference, Society of Automotive Engineers (SAE) Paper 680772 (1968).
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| 7 |
See, e.g., Robertson, supra, at 2. (Statistical review of the U.S. Government's Fatal Accident Reporting System (FARS) for years 1981-1984 shows that vehicles with lowest static stability ratios were associated with highest crash rates; factors such as blood alcohol concentration of the driver, road surfaces, prior license suspensions, and the like failed to negate these outcomes.)
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| 8 |
See, e.g., R. P. Johnson, In Search of the Right Experts in Products Cases, Trial Magazine, February, 1992, at 36-41.
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| 9 |
See A. B. Kelley, R. P. Johnson, L. S. Eidson, Short on Safety: How Auto Designs Cause Needless Harm, National Trial Lawyer Magazine, November 1992, at 22-34.
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| 10 |
Buccery v. GM Corporation (1976) 60 Cal.App.3d 533, 540-41; and 1966 National Traffic and Motor Vehicle Safety Act, 15 U.S.C. § 1397(c) Public Law 89-563, Subsection 108(c). See also Larsen v. General Motors Corporation, 391 F.2d 195 (8th Cir. 1968).
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| 11 |
Barker v. Lull Engineering Co. (1978) 20 Cal.3d 413, 436, 143 Cal.Rptr. 225, 239. |
