The U.S. Supreme Court has reinstated a wrongful death lawsuit that claims Mazda was negligent in not equipment all vehicle seats with lap-and-shoulder belts instead of simple lap belts.

The case involves the death of Thanh Williamson, who was killed when her family's 1993 Mazda minivan was struck head-on by a Jeep that broke loose from a motor home that was towing it.

Ms. Williamson had been sitting in a middle-row seat that was equipped with only a lap belt. Other members of her family were sitting in seats that had lap-and-shoulder belts. She was killed; they survived.

Federal regulations at the time gave vehicle manufacturers the option to use lap belts in certain inner seats in middle and rear rows.

A California state court had dismissed the lawsuit, finding that federal regulations superseded state laws.

The Supreme Court justices were unanimous in their decision that the Williamsons' lawsuit was not pre-empted since "providing manufacturers with this seatbelt choice is not a significant objective of the federal regulation."

Writing for the court, Justice Stephen Breyer noted that the Transportation Department's (DOT) goal in giving automakers a choice about rear seat belts was based only on the concern that lap-and-shoulder rear belts would not be cost-effective.

"But that fact - the fact that DOT made a negative judgment about cost effectiveness - cannot by itself show that DOT sought to forbid commonlaw tort suits in which a judge or jury might reach a different conclusion," Breyer wrote, noting that DOT expected costs to drop with innovation.