An Idaho State Police investigation has found that excessive speed probably caused an Air Force transport truck carrying a Minuteman II missile to flip on its side two weeks ago as it left I-15 for an eastern Idaho weigh station.

The civilian driver, Barry Hatch, 35, of Salt Lake City has been cited for driving too fast for conditions, authorities said.According to the police report, investigators found no evidence of pre-accident mechanical malfunctions that would have caused the truck to overturn on April 18, closing the interstate for extended periods on three straight days.

Investigators determined that the tractor and the enclosed trailer carrying the missile was traveling just under 60 mph as it approached the Sage Port of Entry on a ramp road posted at 25 mph.

The port of entry is the only building for miles along the desolate stretch of highway through the eastern Idaho desert about 25 miles north of Idaho Falls,

The missile did not include the warhead or guidance systems, but the accident forced closure of the interstate for about seven hours until Air Force experts determined there was no danger of the solid fuel rocket exploding. The road was shut down again for about three hours on each of the two following days as Air Force personnel removed the rocket.

Troopers said Hatch, who was hospitalized in Idaho Falls for one day with minor injuries, told them the truck's speedometer and tachometer were not working properly.

The investigative report said the truck's left tires began making impressions in the asphalt road as the vehicle entered the righthand curve. The tire prints continued for 400 feet until the truck rolled onto its left side, sliding 22 feet on asphalt and another 27 feet on gravel and dirt.

The State Police calculated the truck traveled at 26 mph after rolling onto its side.

The vehicle was taking the missile from Great Falls, Mont., to Hill Air Force Base north of Salt Lake City for routine maintenance.

The Air Force has yet to release an estimate of damage to the Minuteman II missile. Damage to the tractor-trailer was put at $9,000.