In a unanimous decision, The United States Supreme Court has determined that car manufacturers are subject to suit based upon their failure to install shoulder belts in the rear inner seats of their vehicles.  In the case of Williamson, et al. v. Mazda, the Williamson family brought suit in California state court on behalf of the estate of Thanh Williamson alleging that Thanh died in a motor vehicle accident because the rear inner seat of the Mazda minivan in which she was riding had a lap-only seatbelt as opposed to a lap-and-shoulder belt.

The state trial court dismissed the suit, holding that the Williamson’s claim was pre-empted by a  federal regulation that gives car manufacturers the option of installing lap-only seatbelts or lap-and-shoulder belts in rear inner seats.  The State Court of Appeal affirmed this ruling relying upon the case of Geier v. American Honda Motor Co., in which it was held that federal regulation requiring passive restraints pre-empted a state tort suit against an auto manufacturer for failure to install airbags.

The U.S. Supreme Court overturned this ruling holding that the only way that Mazda would be immune from suit is if the "significant objective" of the federal regulation at issue was to give auto manufacturers a choice of which seat belts to install.  The evidence introduced indicated that the Department of Transportation’s decision not to require manufacturers to install lap-and-shoulder belts was purely cost driven, having no relation to the safety goals of the regulation.