About 4:00 p.m. e.d.t., on May 12, 1977, a partially loaded flatbed truck, towing a flatbed utility semitrailer fully loaded with a loader-backhoe tractor, was descending a curved, steep, long grade near Marion, North Carolina, when the combination vehicle suddenly began to accelerate. The truck-semitrailer remained on the roadway for about 1/3 of a mile and was negotiating a curve when it skidded sideways; the semitrailer crossed into the opposing traffic lane. The loader-backhoe tractor broke free of its restraining chains and vaulted to the roadway into the path of an oncoming van. The truck-semitrailer continued ahead, left the road, and struck a tree. Five occupants of the van were killed and two were critically injured. The truckdriver was not injured; a passenger in the truck received minor injuries.