ARLINGTON, VA - In the first Insurance Institute for Highway Safety roof strength tests of midsize SUVs, six drew the top rating of "good" for rollover protection, one was deemed "acceptable," and five others were judged "marginal." 

Midsize SUVs drawing "good" ratings from IIHS were the 2010 Chevrolet Equinox (twin GMC Terrain) built after March 2010, Jeep Liberty (twin Dodge Nitro), Toyota Highlander and Venza, plus the Jeep Grand Cherokee and Kia Sorento, both 2011 models. 

The 2010 Ford Edge was rated "acceptable." The vehicles given "marginal" ratings by IIHS were the Honda Accord Crosstour, Honda Pilot, Mazda CX-7, Mitsubishi Endeavor, and Nissan Murano -- all 2010 models. 

In addition to drawing "good" ratings for rollover protection, the Equinox, Grand Cherokee, Highlander, Sorento, and Venza also won the IIHS's Top Safety Pick award. To achieve this, a vehicle must earn "good" ratings for occupant protection in front, side, rear and rollover crashes. The vehicle also must have electronic stability control. 

The rollover rating system is based on IIHS research showing that occupants in vehicles that roll benefit from stronger roofs, the institute said. Vehicles rated "good" must have roofs that are more than twice as strong as the minimum required under the current federal safety standard. The ratings, products of the institute's roof-strength testing program, add to consumer information tests that rate vehicles' front, side and rear crashworthiness. The rollover test is designed to help consumers pick vehicles that will protect them the best in one of the most serious kinds of crashes, IIHS said. 

"Midsize SUVs are a big group so we're testing them in stages," said IIHS President Adrian Lund.

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Top performance in the roof test is important because nearly 10,000 people a year are killed in rollover crashes, IIHS said. When vehicles roll, their roofs hit the ground, deform and crush. Stronger roofs crush less, reducing injury risk from contact with the roof itself. Stronger roofs also can prevent people, especially those who aren't using safety belts, from being ejected through windows, windshields or doors that have broken or opened because the roof deformed. Roofs that don't collapse help keep people inside vehicles when they roll. 

Of course, the best occupant protection is to keep vehicles from rolling in the first place. Electronic stability control is significantly reducing rollovers, especially fatal single-vehicle ones. When vehicles roll, side curtain airbags help protect people. Safety belt use is essential. 

In the IIHS roof strength test, a metal plate is pushed against one corner of a roof at a constant speed. To earn a "good" rating, a roof must withstand a force of four times the vehicle's weight before reaching five inches of crush. For an "acceptable" rating, the minimum strength-to-weight ratio that's required is 3.25. A "marginal" rating value is 2.5, and anything lower than that is deemed "poor." 

The Grand Cherokee, Highlander, Liberty and Venza withstood forces of nearly five times their weights. 

A strength-to-weight ratio of four reflects an estimated 50-percent reduction in serious or fatal injury risk in single-vehicle rollover crashes, compared with the current federal standard of 1.5, IIHS said.