The fortwo was the only 2009 model year minicar to earn the top rating of good in test results released by the Institute. The Honda Fit, Hyundai Accent, Mini Cooper and Toyota Yaris received the second-highest mark of acceptable. The 2009 Chevrolet Aveo got the second-lowest score of marginal.
More than 10,000 people are killed in rollover crashes in the United States each year. The Obama administration issued new regulations in May on vehicle roof strength requirements, the first significant upgrade of the rules since 1973.
With the changes, vehicles need to withstand direct pressure of three times the weight of the vehicle, compared with the previous requirements of 1.5 times the vehicle's weight. The regulations will be phased in beginning in September 2012 and will affect all vehicles by the 2017 model year.
The institute testing is more demanding. It requires a vehicle's roof to withstand a force of four times the vehicle's weight to receive the top score. For the second-highest rating, a roof must withstand a force of 3.25 times the vehicle weight. The second-lowest score applies to vehicles with a strength-to-weight ratio of 2.5, and anything lower receives the worst rating.
Tests results found the fortwo had the highest strength-to-weight ratio of 5.41, followed by the Yaris with 3.78, Accent with 3.72 and the Mini Cooper with 3.44. The Fit's strength-to-weight ratio was 3.42, and the Aveo's was 3.09.
"The federal government's leisurely phase-in of the new standard means roofs won't have to get stronger right away, so we plan to continue rating vehicle roof strength for the foreseeable future," institute president Adrian Lund said.
GM spokeswoman Janine Freuhan said "one test alone does not determine whether a vehicle is safe." She said all GM vehicles, including the Aveo, meet government safety standards.
Dave Schembri, president of smart USA, said it was the latest standardized crash test performed by the government or the insurance industry that confirms the fortwo's safety features.
"As our society increasingly turns to small cars, Smart proves that small can be safe," Schembri said.