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ARLINGTON, Va. — When it comes to building roofs that protect occupants in the event of a rollover, some automakers have excelled, but there is still some room for improvement, according to the Insurance Institute for Highway Safety, which recently conducted its first roof strength tests on a dozen midsize SUVs.

Of the models that IIHS evaluated, six midsize SUVs were deemed to have a "good" rating for rollover protection — which is the highest score — while five were found to be "marginal" (the second-lowest rating) and one vehicle's rollover protection was determined to be "acceptable."

The ratings of the 12 midsize SUVs are as follows:

Good: 2010 Chevrolet Equinox (twin GMC Terrain) built after March 2010, Jeep Liberty (twin Dodge Nitro), Toyota Highlander and Venza, as well as the 2011 model-year Jeep Grand Cherokee and Kia Sorento.

Acceptable: 2010 Ford Edge.

Marginal: 2010 model-year editions of the Honda Accord Crosstour, Honda Pilot, Mazda CX-7, Mitsubishi Endeavor and Nissan Murano.

"Midsize SUVs are a big group so we're testing them in stages," says Institute president Adrian Lund. "First results show that automakers are making progress in rollover protection, but it's disappointing that a new design like the Crosstour didn't perform better."

IIHS also gave the Equinox, Grand Cherokee, Highlander, Sorento and Venza its Top Safety Pick designation. To earn this distinction, vehicles must be rated "good" for front-, side-, rear- and rollover-crash protection, and include electronic stability control.

The Institute said it bases its rollover rating system on its research indicating that stronger roofs protect vehicle occupants better during rollovers. To earn a "good" score in this particular element, the roof of the vehicle must be two times as strong as the minimum federal safety standard.

These tests augment the Institute's ratings of front-, side- and rear-crash protection.

And earning high marks it comes to roof protection can be vital, as the number of people killed annually in rollover crashes is close to 10,000.

"When vehicles roll, their roofs hit the ground, deform and crush. Stronger roofs crush less, reducing injury risk from contact with the roof itself," IIHS officials explained. "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."

Rollover prevention is the most ideal solution, though.  What has been extremely helpful in decreasing rollover accidents — particularly one-vehicle wrecks — is electronic stability control.

During the event of a rollover, side-curtain airbags also serve as important protection, and wearing a seatbelt is vital, as well.

Explaining the roof tests in more detail, officials noted: "In the Institute's 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 poor," they explained further.

Scores for the Grand Cherokee, Highlander, Liberty and Venza were all close to 5, while the Crosstour was at a 2.8 strength-to-weight ratio and the Endeavor and Pilot were near 3.

If a vehicle scores a 4, it can reduce the risk of fatal injury in single-vehicle rollover crashes by approximately 50 percent versus a vehicle with the score of 1.5, which is the current federal standard.