Rollover / Stability

The tragic consequences of rollovers rose to public prominence in the 1980s. A series of studies showed that utility vehicles experienced a disproportionate number of rollovers compared to passenger cars. The influential Insurance Institute for Highway Safety (IIHS) reported in 1980 that utility vehicles were rolling over at five times the rate of passenger cars. And accident data from the National Highway Traffic Safety Administration (NHTSA) found that death and disabling injuries were twice as high in utility vehicles compared to passenger cars. In 2003, 281,000 vehicles rolled over – 20 percent resulted in fatalities and 170,000 resulted in injuries.
At the time, the population of light trucks on the road was also increasing. In 1975, light trucks – including pick-up trucks, minivans and SUVs – accounted for 18 percent of all passenger car and light truck vehicle registrations in the U.S. In 2004, the percentage of light trucks had risen to 40 percent of total passenger vehicles.
Despite the mounting evidence of light trucks’ stability problems and their popularity with consumers, NHTSA has not established a standard for vehicle stability. The last quarter century has been devoted to an ongoing debate among the federal agency, researchers, automakers and safety and consumer advocates over the best way to measure a vehicle’s propensity to roll and whether reductions in rollover fatalities could be best achieved by more driver education, improved rollover occupant protection – restraints, glazing; rollover air bags – or better vehicle design.
Manufacturers have fought hard against any attempt to promulgate a rollover stability regulation or even caution motorists about a vehicle’s propensity to rollover. To date, the agency has acquiesced to industry on the core issue of a stability standard. Instead, NHTSA has settled for rollover warning regulations and a rating system.
The Early History of the Vehicle Stability Standard
The history of the regulations on roll stability standard began in 1970, when the National Highway Safety Bureau, NHTSA’s predecessor, proposed an amendment on passive occupant protection that included a rollover compliance that would subject a vehicle to two complete rollovers between 30 mph and 60 mph with the windows open, and require that anthropomorphic test dummies be retained in the vehicle.
Manufacturers protested the provision, arguing that no two rollover accidents were alike, making it impossible for them to design a restraint system that would protect occupants in all scenarios. The industry also argued that there were no rollover test procedures that produced reliable and reproducible results. For two years, industry mounted a campaign against the regulation, meeting with the agency and filing petitions for reconsideration.
NHTSA’s own testing, however, produced a high degree of repeatability. In 1971, the agency issued a final rule amending FMVSS 208 to require passenger cars manufactured on or after August 15, 1975, and light trucks and multipurpose passenger vehicles manufactured on or after August 15, 1977, to comply with a dolly rollover test requirement.
Shortly after issuing that rule, NHTSA established a new standard FMVSS 216 – Roof Crush Resistance. This new rule set a minimum strength requirement for the roofs of passenger cars to reduce the likelihood of collapse in rollover accidents. FMVSS 216 was meant to be a temporary alternative to the passive occupant protection requirements in the rollover portion of FMVSS 208. Manufacturers could meet the dynamic rollover standard of FMVSS 208 or the much less stringent static roof crush standard under FMVSS 216 until August 1977, when FMVSS 216 was to be revoked.
The agency failed to revoke FMVSS 216. It is still the standard manufacturers use to meet compliance. NHTSA also dropped its effort to establish a stability standard. In 1973, after postponing the compliance date of the FMVSS 208, NHTSA, without explanation, let the rollover test requirement die.
NHTSA Changes Approach to Rollover Safety
In the Reagan era, the agency took a new tack in mitigating rollover hazards. It began to focus its criticisms on driver behavior instead of automobile design. Rather than ensuring that vehicles were more stable, the agency opted for regulations that would warn consumers about which vehicles were not stable.
In 1982, it proposed adding a warning-label requirement for utility vehicles that had “special features” for occasional off-road operation. Finalized in May 1984, the new rule required manufacturers to place warnings in short-wheelbase utility vehicles to alert drivers that these vehicles handle and maneuver differently from passenger cars and to include information in the owner’s manual concerning the proper method of on-and off-road driving. This regulation did little to solve the problem.
In the latter half of that decade, the data showed on-road rollover rates were climbing. One influential study, “The Role of Stability in Rollover-Initiated Fatal Motor Vehicle Crashes Under On-Road Driving Conditions” (Leon Robertson, Ben Kelley) concluded that vehicle makes and models with the lowest stability values had rollover fatal crash rates much greater than those for other vehicles; that the lowest stability values were associated with the highest crash rates; and that the introduction of other major risk factors in the analysis failed to negate that outcome.
The Robertson / Kelley study gave prominence to a calculation known as the Static Stability Factor. The Static Stability Factor (SSF) is a simple calculation: half the vehicle’s track width divided by its center of gravity – T/2H. The lower the SSF, the higher the risk of a vehicle rolling over. Robertson / Kelley’s research showed that a vehicle’s SSF was a statistically robust metric in determining likelihood of a rollover.
At the same time, consumer advocates began clamoring for more effective regulations than a warning label. Consumer’s Union, the Center for Auto Safety and then-Colorado Congressman Timothy Wirth all petitioned NHTSA on this emerging issue. They either requested that the agency develop some sort of stability regulation or to initiate a defect investigation into “open-top Jeep-type vehicles,” and the Samurai and Samurai variants, the SJ410 and LJ80.
The agency eventually denied all the requests for defect investigations and Wirth’s request for a standard requiring that vehicles have a minimum Static Stability Factor as a way to ensure vehicles were less prone to rollover. At the time, the agency acknowledged that a vehicle’s SSF had some relation to its overall involvement in rollover incidents. But, it said, basing an effort to address the rollover problem on the stability factor alone was too narrow and an inappropriate approach.
In 1988, NHTSA did grant Consumers Union request to begin a rulemaking to establish a minimum stability standard. Four years later, the agency formally announced that it was again considering establishing federal standard to reduce rollover fatalities. Among the options on the table were: improving vehicle stability and reducing rollovers, improving crashworthiness/occupant protection in the event of a rollover, and providing consumer information on a vehicle’s rollover propensity.
But in 1994, NHTSA once again halted the effort to develop a vehicle stability standard. “No single type of rulemaking or agency action could solve all, or even a majority of, the problems associated with rollover,” the agency asserted in a Federal Register notice. NHTSA noted that it needed to identify the vehicle metrics that had a causal relationship to the likelihood of rollover and a statistical relationship to rollover frequency.
Instead, the agency again sought to offer some protection to occupants during a rollover by tinkering at the margins of the roof crush resistance standard. In November 1989, NHTSA proposed extending FMVSS 216 Roof Crush Resistance to light trucks, vans, and buses and to eliminate the 5,000 pound test force limit for light trucks. The agency observed a significantly greater incidence of rollover crashes, per registered vehicle, involving light trucks as compared to passenger cars. Since most light truck manufacturers were already voluntarily complying with FMVSS 216, there was little resistance to extending the standard. The final rule, published in April 1991, extended FMVSS 216 to light trucks with a GVWR of 6,000 pounds or less.
Consumer Warnings Replace Stability Standard
Throughout the 1990s, the agency continued to focus on consumer education as the way to reduce rollover fatalities. In a 1996 status report, the agency argued that rollovers were complex events that could not be completely eliminated, and that a major reduction in rollover fatalities would require a multi-disciplinary approach: safety labels to help consumers choose a vehicle, more research on dynamic rollover characteristics and on the effects of ABS, improved rollover occupant protection, better seat belts, seat belt usage information and requirements.
On the same day in 1994 that NHTSA dropped the development of a stability standard, the agency proposed a requirement that vehicles with a short wheelbase carry a rollover resistance label. Automakers, of course, applauded the decision to forego a stability standard; safety advocates were scathing in their criticisms of the aborted rule.
In support of its rollover information approach, the agency studied ways to measure vehicle stability with dynamic tests – as opposed to the Static Stability Factor. In May 1997, NHTSA began its first phase of a multi-phase program to examine on-track handling maneuvers. This first phase was an exploratory study that examined maneuvers to quantify on-road, untripped rollover propensity. Researchers examined eight test procedures, including J-turn (without pulse braking), J-Turn with pulse braking, brake and steer, steering reversal, Toyota Fishhook (without pulse braking), double lane change, split-mu two wheels off-road recovery simulation and Toyota fishhook with pulse braking.
In July 1999, the NHTSA released a report on its test track research on which rollover maneuvers that might be most useful for identifying potential stability problems. NHTSA’s test arm, the Vehicle Research and Test Center (VRTC), performed two types of tests. The first determined the fundamental vehicle handling properties (pulse steer, sinusoidal sweep, slowly increasing steer/slowly increasing speed). The second test measured rollover propensity (J-turn, J-turn with pulse braking, two fishhook tests with different reversal timings and steering rates, and a resonant steer test). Comparing the test results to vehicle metrics, NHTSA found that the steering maneuver and pulse braking scoring methods were better related to static rollover metrics (Static Stability Factor and Tilt Table Ratio) than the dynamic Critical Sliding Velocity measure. They also found that the J-turn appeared to be a “coarse” metric to discriminate between vehicles with high or low rollover propensity.
During this decade, there was little regulatory activity. In 1997, the agency granted and then, later that year, denied another Consumers Union petition to establish a rule creating an emergency-handling standard for SUVs .
Meanwhile, manufacturers continued their fight to weaken the warning label regulations. They protested that the proposed consumer information standard stemmed from a single metric, which, they claimed, was not representative of real-world crash modes. Some, like GM, argued that a rollover label would not likely improve safety and that vehicle design was the least prominent factor contributing to rollover accidents.
Nonetheless, the agency was able to finalize an amendment to the regulations governing the consumer warning label required on Multi Purpose Vehicles with a wheelbase of 110 inches or less. The new final rule, published in March 1999 required a modified rollover-warning label for SUVs. The new brightly colored label affixed to the sun visor or the driver’s side window stated: “Warning! Higher Rollover Risk;” “Avoid Abrupt Maneuvers and Excessive Speed;” “Always Buckle Up” and depicted a vehicle tipping and a belted occupant. The rule also required more rollover risk information to be included in the owner’s manuals of these vehicles.
“Stars on Cars”
In recent years, roll stability has become a facet of NHTSA’s New Car Assessment Program. This focus has kept the agency off the path of regulating automakers and using its NCAP system to provide consumer ratings. NHTSA began the process of adding a rollover rating to new cars in June 2000. At the time, the rollover problem was worsening, with 9,527 fatalities in 1997 to 10, 856 in1999. The five-star rating system, based on a vehicle’s Static Stability Factor, would award one star to vehicles with an SSF of 1.04 or less and a rollover risk of 40 percent or greater and would award five stars to vehicles with an SSF of 1.45 and a rollover risk of less than 10 percent.
Twelve years after rejecting the SSF as a good stability measure, NHTSA decided to use it because of its good correlation to crash outcomes, applicability to tripped and untripped rollovers, and its accuracy and repeatability, and because changes to improve SSF were likely to have negative consequences. Manufacturers protested using the SSF because it had been rejected many times as a useful tool in predicting rollover.
That argument found a sympathetic ear in U.S Senate Appropriations Committee Chair, Richard Shelby (R, AL). Shelby added language to a DOT appropriations bill designed to delay or kill the rollover rating regulation. The provision prohibited NHTSA from finalizing or implementing the proposed rollover rating regulation until the National Academy of Sciences determined that the SSF was scientifically valid and a rating based on this measure provides some benefit to consumers.
But the Firestone/Ford Explorer rollover debacle of 2001 shifted support on Capital Hill and the delay was lifted. On January 12, 2001, NHTSA finally issued a rule to incorporate a rollover resistance rating for new cars and light trucks into the New Car Assessment Program (NCAP) The agency again determined that consumer information was the best way to reduce the number of rollover injuries and fatalities. NHTSA reasoned that enabling consumers to make choices based on differences in rollover risk would also serve as a market incentive for manufacturers to design more stable vehicles.
In the meantime, the National Academy of Sciences has since bestowed its blessing on the validity of the Static Stability Factor and NHTSA has continued its research into dynamic test methods. In October 2002 NHTSA released the Phase IV report of its research on light vehicle rollovers – one of the agency’s most significant documents on stability. For the first time, the agency tested and identified objective, repeatable maneuvers that discriminated dynamic vehicle rollover propensity and were refined enough to be used for regulatory or consumer information purposes. The results formed the foundation for what officially became the agency’s rollover stability test protocol of its dynamic rollover ratings.
The Recall Enhancement, Accountability and Documentation (TREAD) Act of 2000 required the Secretary of Transportation to develop a dynamic test for rollover resistance for use in a consumer information standard. In October 2003, NHTSA issued its Final Policy Statement modifying NHTSA’s NCAP rollover ratings to include dynamic rollover testing.
Has any of this made a difference? NHTSA’s most recent fatality statistics showed that 10,816 people died in rollover crashes in 2004 – about the same number of fatalities as in 1999. But vehicles are becoming more stable. The latest crop of cross-over SUVs shows that this class of vehicle is becoming less like a truck and more like a passenger car. These changes were likely driven more by consumer demands for more comfort and convenience than by the government ratings.
Perhaps the agency’s most far-reaching roll-stability regulatory action to date has been a proposal that would not make vehicles more structurally stable, but prevent them rolling over in the first place. [See "A Brief History of Electronic Stability Controls and their Applications"] In September 2006, NHTSA proposed to make electronic stability control systems mandatory on all passenger vehicles 10,000 pounds or less, beginning in September 2008. The new standard, Federal Motor Vehicle Safety Standard 126, would be phased in over a five-year period, with the entire fleet equipped by 2011. Electronic Stability Control has been available in the U.S. since 1997. But during the last decade, manufacturers have made little progress in spreading this life-saving technology. Only 29 percent of U.S. light vehicles are so equipped, compared to 80 percent of their European counterparts.
One of the key factors driving the regulation in the U.S. is a statistical analysis prepared by the agency demonstrating ESC’s remarkable effectiveness. An analysis of 1997-2004 accidents in the Fatality Analysis Reporting System and 1997-2003 crash data from state files shows that ESC significantly decreased run-off-road, fatal single vehicle and culpable multi-vehicle fatal crashes. For example, ESC reduced fatal run-off-road crashes by 35 and 72 percent for passenger cars and light trucks, respectively. ESC helped drop the number of fatal single-vehicle crashes by 35 percent in passenger cars and 67 percent in light trucks. Overall, the researchers concluded, ESC reduced fatal crashes in passenger cars by 14 percent and by 29 percent in light trucks. The researchers noted, however, that ESC is primarily limited to luxury models of passenger cars and SUVs. The technology in the U.S. has sparsely applied to minivans and trucks.
Copyright © Safety Research & Strategies, 2006