The Smart Fortwo has the strongest roof and the Chevrolet Aveo has the weakest among 2009 micro and minicars recently tested by the Insurance Institute for Highway Safety. The Smart earns the highest rating of good compared with acceptable for the Honda Fit, Hyundai Accent, Mini Cooper, and Toyota Yaris. The Aveo is rated marginal.   The rating system is based on Institute research showing that occupants in rollover crashes benefit from stronger roofs. Vehicles rated good must have roofs that are more than twice as strong as the current minimum federal safety standard requires. The ratings, products of the Institute’s new roof strength testing program, add to consumer information tests that rate vehicles for front, side, and rear crashworthiness. The roof test is designed to help consumers pick vehicles that will help protect them in rollover crashes.