DETROIT — Government regulators have tested more than 100 automobile models for their tendency to roll over and found that sport utility vehicles tend to be the least stable, particularly when fully loaded, while cars are the most stable.
After 27 years of controversy over the rollover issue, members of the House and Senate Appropriations Committees were meeting to approve a transportation budget that would let the National Highway Traffic Safety Administration begin rating vehicles' rollover tendencies, by assigning them one to five stars.
But safety regulators have already calculated stability scores for many models through the 1998 model year, so as to devise the rating systems. These scores, along with the method for converting them into stars, are already in the agency's public docket.
While most cars receive four stars under the system, and some receive five, most sport utilities have scores that will net them just two stars.
The government plans to publish the data in December for vehicles that are empty except for a driver. But government test data show that many sport utilities would receive only one star if loaded with an adult in each seat and cargo equal to one or two suitcases.
By contrast, the scores for sedans are virtually unaffected by the number of people and amount of cargo inside. People sit fairly low in sedans, keeping them stable, but the seats in sport utilities are mounted high off the road, in what auto marketers call the "command position," and this makes the off-road vehicles more prone to tip over when fully loaded.
The method for calculating the ratings — comparing the width between a vehicle's tires to the height of its center of gravity — is controversial. Automakers complain that it does not take into account the electronic stability systems now available on a few luxury models, nor does it reflect seat belt usage, the best way to avoid dying or being paralyzed in a rollover crash. Consumers Union, the publisher of Consumer Reports, complains that by lumping most sport utilities together near the bottom of the rankings, the method does not allow consumers to distinguish more stable sport utilities from the less stable.
A few sport utilities have become a little less unstable in recent years, the government data show. A key exception is the Ford Explorer, which has accounted for most of the 103 deaths now blamed on Firestone tires, mainly because the Explorers rolled when tires lost their treads.
The Explorer received a score of 1.08 for the 1991 to 1994 model years, but slipped to 1.06 for 1995 to 1998, according to the government's calculations. Both scores would qualify the Explorer for just two stars, which encompasses scores from 1.05 to 1.12. A rating of two stars means that a vehicle has a 30 to 40 percent chance of rolling over during a single-vehicle crash that merits a police report.
Ford has calculated from government data on fatal crashes across the nation that Explorer occupants are 24 percent less likely to die in fatal crashes than are occupants of other midsize sport utility vehicles. But insurance industry data also show that Explorer occupants are almost twice as likely to die in rollovers as occupants of Ford Taurus sedans.
By making sport utilities like the Explorer heavier over the years, automakers have reduced somewhat their propensity to roll over and allowed them to override the bumpers and door sills of cars they hit, thereby transferring to other motorists much of the risk once associated with driving a sport utility. Overall death rates for sport utility occupants are now no higher than for car occupants.
The most popular minivan models receive three stars under the government rating system, while pickup trucks receive one to three stars. Publicly available test data run through the 1998 model year; some models have changed since then, and some sport utilities are being widened to make them more stable, including the 2002 Explorer, which is slated to go on sale early next year.
Vehicles' susceptibility to the loss of a tire varies with their height, auto industry engineers say. This is because the taller the vehicle, the more its weight tends to move forward during braking, making the driver very dependent on maintaining whatever traction possible on the rear wheels to maintain control. If one of the rear tires fails — Ford says the left rear tire seems the most prone to failure on the Explorer — then controlling the vehicle becomes much harder. This is why groups like American Automobile Association advise motorists to avoid hitting the brakes hard or swerving when a tire fails, particularly if they are driving SUVs.