About 12:40 p.m., e.d.t on September 6, 1985, a 1902 GMC 2-axle truck fitted with a 1973 MC-331 cargo tank overturned while traveling southbound on the Capital Beltway, I-95, near Largo, Maryland. The 2,500-gallon capacity cargo tank contained about 1,375 gallons of propane. The Poist Gas Company truck was traveling between 50 and 55 mph when, according to the drivers the steering wheel started shaking violently and "flew out of my hands." The driver stated that he took his foot off the accelerator but did not brake because be believed that one of the vehicie's tires was experiencing a blow out. The truck veered across the right paved shoulder of the highway and onto a grass shoulder. It then traveled 300 feet down the grass shoulder until the driver steered the truck back to the left to avoid hitting a tree. The truck slipped the north end of a guardrail when it reentered the paved shoulder of the highway as the driver tried to regain control of the truck. The truck then traveled 510 feet down the paved shoulder and the right travel lane of the highway before rotating clockwise about 80° and overturning on Its left side. The vehicle continued to rotate another 100° as it slid 400 feet down the highway on Its left side. The truck came to rest facing north (180° opposite its original direction of. travel) with the top of the cargo tank parallel with and against the guardrail At the time of the accident the roadway was dry and the weather was clear.