"We haven't a clue as to the man's identity at this time," UHP Spokesman Duane Baird said.
The man, who appears to be in his 20s, apparently walked off the shoulder of the freeway at about 700 South Wednesday night, straight into the path of an oncoming semi-truck, Baird said.
There were no cars off the road in the area to indicate the man was seeking help.
He had no identification on him. "We may never know why he walked in front of the semi," Baird said.
The driver of the semi told investigators he did not see the man until it was too late.
The driver was not injured and no one else was involved in the crash.
Update: The UHP later reported that the man's family members identified him as having a long history of mental illness and believe he was suicidal.