An Orem man died instantly Friday when he was thrown onto the highway from the sleeper compartment of a semitrailer truck as it rear-ended another truck parked along the side of the road.

Chester L. Allen, 44, was killed in the accident, which forced the closure of northbound lanes of I-15 north of Malad on Friday.The truck in which Allen was riding was owned by Liquid Air Co. of Salt Lake City, Idaho State Police Capt. Mike Fosbury said. The truck, driven by Sidney W. Lewis, 42, of Kamas, Utah, was traveling north about six miles north of Malad shortly after 6 a.m. when it ran into the back of a disabled truck parked along the right shoulder of the interstate.

"We have no idea why he ran into him," Fosbury said, since emergency triangles had been properly placed behind the disabled truck.

An investigation was continuing.

Lewis suffered broken ribs, a dislocated knee and cuts and bruises in the crash and was taken by Life Flight helicopter to Bannock Regional Medical Center in Pocatello.

The Liquid Air truck was carrying a tanker filled with compressed liquid nitrogen, which overturned. But Fosbury said the contents were venting safely into the air and there was no health or environmental hazard.

He said two Liquid Air officials were flying to the scene from San Francisco to determine how best to remove the liquid nitrogen. In the meantime, traffic was being diverted at Malad over a county frontage road to Downey.

The disabled truck, owned by Chemopharm of Salt Lake City, was operated by Mark Rice, 22, of West Salt Lake City.