Background

Consumers Union (CU) petitioned The National Highway Traffic Safety Administration (NHTSA) in 1996 to develop and conduct dynamic tests for vehicle rollover propensity and to publish the results for consumers. Simply put, dynamic tests involve measuring vehicle behavior while it is in motion performing a task. NHTSA granted that petition in 1997 and commenced work developing such a test program. In 2001, however, they changed course and published a rollover resistance program based only on each vehicle’s static stability factor (SSF). SSF had been considered earlier as one method of evaluating certain aspects of rollover propensity, in particular, tripped rollover. It is a calculation based on two physical dimensions of the vehicle: center-of-gravity height and track-width (SSF = T/2H, with H being the center-of-gravity height and T being the track-width). Rollovers are considered tripped rollovers when the vehicle slides into a curb or other obstacle and trips over that obstacle. This is the most frequent type of rollover crash. Untripped rollovers, on the other hand, usually occur as a result of emergency steering maneuvers on the roadway and generally do not involve tripping over an obstacle.

CU believes risk estimates based on SSF alone are too coarse a measurement to give an accurate picture of a vehicle's propensity to roll over during emergency steering maneuvers. CU also believes that dynamic tests that evaluate vehicle performance under emergency handling conditions at the limit are an essential component of assessing rollover propensity.

CU’s belief in the importance of dynamic testing was confirmed by Congress in the Transportation Recall, Enhancement, Accountability, and Documentation Act of 2001 (TREAD) (Section 12), which directed NHTSA to develop a dynamic test for rollover that would be the basis for "developing meaningful consumer information." Dynamic testing treats the vehicle as a complete entity, measuring its performance as a whole. Such an approach includes many more variables than simply a formula relating track-width and center-of-gravity height. Dynamic tests reflect the stability of the vehicle in real-world driving maneuvers and take into account the vehicle’s suspension, tires, and the effects of safety features like electronic stability control, which SSF cannot.

New Dynamic Rollover Tests

NHTSA completed its work on developing a dynamic test for rollover propensity and published the details of the test program in the Federal Register on October 14, 2003. The test, named by NHTSA as the Road-Edge Recovery Test, also known as the Fish Hook Test, is a stringent test that determines whether a vehicle will tip up or roll over under rigorous, on-road conditions. CU believes these test results are a useful tool for helping consumers differentiate more-stable vehicles -- i.e., less likely to roll over in emergency maneuvers -- from less-stable vehicles.

The new rollover test results provide a much-needed and welcome tool for consumers. That is indeed good news. Taken alone, they help the consumer compare the likely performance of vehicles undergoing emergency maneuvers. In CU's opinion, vehicles that remain stable in NHTSA’s test (i.e., do not tip up on two wheels) are safer than vehicles that tip up or threaten to roll over. Unfortunately, NHTSA has not yet tested very many models using the new dynamic test, and it may be as long as another year before there is enough tested models for consumers to make useful comparisons.

Moreover, CU has serious misgivings about the manner in which NHTSA now presents rollover information. The new rollover ratings are based on a combination of a vehicle's SSF and the results of its performance in NHTSA's dynamic testing program. This overall rating system takes the form of a five-star system, with five stars indicating the least likelihood of rolling over and one star indicating the greatest. However, while CU believes it is important for consumers to consider the attributes of both SSF and dynamic testing as they evaluate available consumer information on rollover propensity, CU believes the new star rating gives far too much weight to SSF and too little weight to the dynamic rollover tests. Indeed, the new overall ratings distort the dynamic test results in such a way as to make vehicle comparisons confusing and even misleading.

In the end, it is the consumer’s choice of which vehicle to buy, and that choice is now better informed with the addition of the new dynamic rollover test information. In CU's opinion, the most prudent approach is to consider vehicles with the highest SSF that also passed NHTSA’s dynamic test -- i.e., vehicles that did not tip up. To learn a vehicle’s SSF and dynamic test results, you simply click on the name of the vehicle in the summary list. (see http://www.nhtsa.dot.gov/ncap/) SSF notwithstanding, CU will not recommend any vehicle that fails NHTSA’s dynamic rollover test.