On May 31, 2001, about 3:28 RM. Central Daylight Time, a southbound Gayle Stuart Trucking, Inc., truck-tractor semitrailer exited Interstate 540 at State Highway 282 near Mountainburg, Arkansas. The driver was unable to stop at the stop sign at the bottom of the ramp. The 79,040- pound combination unit was traveling at approximately 48 mph when it entered the intersection and collided with the right side of a westbound vehicle, a 1990 Blue Bird Corporation 65- passenger school bus operated by the Mountainburg, Arkansas, and Public Schools. The school bus rotated approximately 300 degrees clockwise and overturned; the body, which partially separated from the chassis, came to rest on its right side on the eastbound shoulder of State Highway 282. The tractor semi-trailer continued across the roadway, rotated about 60 degrees clockwise, overturned, and came to rest on its left side. Three school bus passengers seated across from the impact area were fatally injured; one was partially ejected. Two other passengers (one of whom was seated in the impact area) received serious injuries, and four passengers had minor injuries. The school bus driver and the truck driver both sustained minor injuries. The Safety Board determines that the probable cause of the accident was the truck driver's inability to stop the tractor semitrailer at the stop sign at the bottom of the ramp due to the reduced braking efficiency of the truck's brakes, which had been poorly maintained and inadequately inspected. Contributing to the school bus passengers' injuries during the side impact were incomplete compartmentalization and the lack of energy-absorbing material on interior surfaces