Yesterday the U.S. Supreme Court held in Williamson v. Mazda Motor of America, Inc., 562 U.S. __ (2011), that federal regulations do not preempt a claim for design defect based on the failure to provide lap-and-shoulder belts for all rear seats in a passenger vehicle. The opinion, delivered by Justice Breyer, effectively crafted a case-by-case test for determining whether federal regulations offering finite options to manufacturers preempt state law tort claims. The Court strained to distinguish a nearly identical case and significantly raised the standard for a finding of preemption under a federal regulatory scheme. Seven justices joined the majority opinion, with Justice Sotomayor concurring (Justice Kagan took no part).

In 2002, Thanh Williamson was involved in a head-on collision while riding in a Mazda minivan. She was seated in the rear of the vehicle wearing only a lap belt, and died in the accident. Two other rear-seat passengers were wearing lap-and-shoulder belts and survived. Her heirs sued Mazda in California state court, claiming that Mazda should have installed lap-and-shoulder belts on all rear seats, and that she died because her seat was only equipped with a lap belt.