| Introduction and Background
Rollovers are among the most severe traffic crashes, and are of particular concern for occupants of light trucks and vans (LTVs) - including pickup trucks, sport utility vehicles (SUVs), minivans and full-size vans up to 10,000 pounds gross vehicle weight rating (GVWR). While only about 3 percent of all passenger vehicle (passenger car and LTV) crashes involve rollover, according to the 2003 Fatality Analysis Reporting System (FARS), one-third of all passenger vehicle occupants who lost their lives were in vehicles that rolled over, a total of 10,376 rollover deaths. Of these, 4,433 were in passenger cars, 2,639 in SUVs, 2,569 in pickup trucks, 724 in vans, and the remaining 11 in other or unknown types of light trucks. Passenger cars had the lowest rollover fatality rate (23 percent of fatalities were in vehicles that rolled over), while SUVs had the highest, 59 percent. Similarly, according to the General Estimates System (GES), 6 percent of passenger car occupants who were injured were in vehicles that rolled over. LTV rates of rollover-related injured occupants were higher – 9 percent of those injured in vans, 13 percent in pickup trucks, and 20 percent in SUVs. Looking at occupant fatalities per 100,000 registered vehicles, passenger cars had the lowest rate at 3.69, with vans similarly low at a rate of 3.83. The rates for pickup trucks (7.18) and SUVs (10.22) were much higher.1 Clearly, rollover crashes are a major safety problem for all classes of light vehicles, particularly LTVs. On June 1, 2000, the National Highway Traffic Safety Administration proposed adding a measure of rollover resistance to the NCAP program, to begin in model year 2001.2 It was the agency’s belief that consumer information on the rollover risk of passenger cars and LTVs would influence some consumers to purchase vehicles with a lower rollover risk, and inspire manufacturers to produce vehicles with a lower rollover risk. This, in turn, would reduce the number of injuries and fatalities from rollover. Section 12 of The Transportation Recall Enhancement, Accountability, and Documentation (TREAD) Act,3 enacted November 1, 2000, directed the Secretary of the Department of Transportation to develop a rollover test for motor vehicles, to carry out a program of rollover tests, and to develop and disseminate consumer information on rollover performance.4 This responsibility was subsequently delegated to NHTSA. As planned, in model year 2001 NHTSA began including rollover resistance information in its NCAP. The original rollover resistance ratings were determined solely from the vehicle’s static stability factor. The SSF of a vehicle is an at-rest calculation of its rollover resistance, based on its most important geometric properties. Basically, SSF is a measure of how top-heavy a vehicle is. A vehicle's SSF is calculated using the formula:
where 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 average 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 tripped single-vehicle crash. A higher SSF value equates to a more stable, less top-heavy vehicle. SSF values across all vehicle types typically range from around 1.00 to 1.50. Most passenger cars have values in the 1.30 to 1.50 range. Higher-riding SUVs, pick-up trucks, and vans usually have values in the 1.00 to 1.30 range.5 Throughout the development of testing procedures, both linear and logistic regressions were run to determine and verify the relationship between SSF and rollover.6 7 8 It was found that the correlation of SSF to rollovers in single-vehicle crashes is exceptionally robust in an area as complex as rollover. Commenters on the Federal Register announcement had suggested that the rating system did not go far enough, and suggested that a dynamic test be added.9 NHTSA sought to expand the information provided in its rollover resistance ratings, and requested comments on the subject of dynamic rollover testing.10 After evaluating numerous driving maneuver tests for the dynamic rollover consumer information, NHTSA published its findings in a notice of proposed rulemaking in the Federal Register.11 The final modifications to the rollover resistance ratings in NCAP, including the addition of dynamic rollover tests, were announced12 and took effect beginning with model year 2004 vehicles. The new dynamic maneuvering test used by NHTSA to help evaluate rollover risk utilizes a heavily loaded vehicle (to represent a five-occupant load), and a full tank of gas. Using a fishhook pattern, the vehicle simulates a high-speed collision avoidance maneuver—steering sharply in one direction, then sharply in the other direction—within about one second. Test instruments on the vehicle measure if the vehicle’s inside tires lift off the pavement during the maneuver ("inside" meaning the left wheels if turning left, and the right wheels if turning right). The vehicle is considered to have tipped up in the maneuver if both inside tires lift at least two inches off the pavement simultaneously. The tip-up/no tip-up results are then used with the SSF measurement as inputs in a statistical model that estimates the vehicle’s overall risk of rollover in a single-vehicle crash.13 Thus, for model years prior to 2004, rollover resistance ratings were based solely on the SSF. Starting with 2004 model year vehicles, rollover ratings combine both SSF and the tip-up or no tip-up results of the dynamic maneuvering rollover test. The results of this new process of generating rollover ratings, and its effect on future SSF trends as well as the design of future vehicle, are outside the scope of this report. They can be evaluated in the future as data becomes available. NHTSA’s Web site and Buying A Safer Car brochure provide rollover resistance information as well as ratings for specific make and models of vehicles by model year. Rollover ratings for individual vehicles can be viewed by visiting www.safercar.gov. Passenger vehicles of all types (cars and LTVs) are included. For simplicity, ratings are converted to a graphical star system, similar to NCAP’s impact testing program. A vehicle’s rollover resistance rating is an estimate of its risk of rolling over in a single-vehicle crash, not a prediction of the likelihood of a crash. As the chart below indicates, the lowest-rated vehicles (1 star) are at least four times more likely to roll over than the highest-rated vehicles (5 stars) when involved in a single-vehicle crash.
Because rollover risk is a function of vehicle design, the different vehicle types tend to have differing patterns of risk. The injury and fatality experience also bear this out, as previously noted. The chart below helps consumers make comparisons across vehicle classes. The information is based on data from all vehicles tested under NHTSA’s 2001-2003 rollover resistance ratings system (SSF only). For example, all passenger cars that were tested have 4- or 5-star ratings, and an average chance of rollover of approximately 12 percent if involved in a single-vehicle crash. The range for passenger cars is approximately 6 percent to 19 percent. The purpose of this report was to track the trend in SSF over several years for numerous vehicles, to determine whether more recent models have higher SSF values and are therefore less prone to rollover. The rollover ratings are produced for consumer information only; no regulatory requirements specifically related to rollover mitigation are placed on vehicle manufacturers. If consumers use the information to purchase vehicles with higher rollover reliability ratings, then manufacturers would presumably design and produce vehicles to meet that public demand. This report attempts to see if that is what has indeed occurred in the marketplace. 12003 Fatality Analysis Reporting System (FARS); NHTSA; USDOT. |
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![static stability chart for a range of vehicle types - click [d] for long description](../images/ssfChart.jpg)