Also earning good ratings in the IIHS test were the Subaru Forester and Honda Element. The Suzuki Grand Vitara, Toyota RAV4, Nissan Rogue and Mitsubishi Outlander were rated acceptable. The Honda CR-V was rated marginal.

The rating system, which is higher than the government standard, is based on the IIHS research showing that occupants in rollover crashes benefit from stronger roofs.

Under the IIHS standard, to earn a good rating, the roof must withstand a force of four times the vehicle's weight before reaching five inches of crush. This is a strength-to-weight ratio. Under the Institutes standard, for an acceptable rating the minimum required strength-to-weight ratio is 3.25. A marginal rating value is 2.5. Anything over than that is poor, IIHS said.

In a statement, IIHS President Adrian Lund said the strength-to-weight ratio of four reflects an estimated 50 percent reduction in risk of serious and fatal injury in single-vehicle rollover crashes compared with the current federal standard of 1.5. The federal standard has not changed since 1973.

"We anticipate that our roof strength test will drive improved rollover crash protection the same way that our frontal offset and side impact consumer test programs have led to better protection in these kinds of crashes," said Mr. Lund.

The IIHS said that more than 10,000 people a year are killed in rollover accidents. According to the Institute, stronger roofs can prevent occupants from being crushed by the roof or being ejected from their vehicles because windows, windshields and doors are not broken.

Tests indicate roofs have gotten stronger over the past few years, the IIHS said. The organization said it believes that car design improvement to better withstand front and side structural collisions has made the difference. Both the Volkswagen and Subaru, which earned good ratings in the roof test, were the first to ace the IIHS front and side tests, Mr. Lund noted.

The National Highway Traffic Safety Administration (NHTSA) has proposed a new standard, but not as tough as the IIHS' recommendation. The NHTSA argues that upgrading the requirements would have only a limited impact on rollover deaths and injuries, while the IIHS said its research indicates otherwise

The IIHS said the roof strength rating will be a new requirement to earn its Top Safety Pick award for 2010. Because of the added requirement there will be fewer winners in 2010, Mr. Lund predicted.

The IIHS said it plans to assess the roof strength on mini-cars and midsize cars next.