James William Flynn, 29, ran from security personnel at the hospital on 1600 W. Antelope Drive and into the nearby southbound lanes of the Antelope Drive off-ramp about 6:20 p.m., according to the Utah Highway Patrol.
Flynn dodged vehicles as he ran onto the outside southbound lane of I-15, where he was hit by two vehicles, said UHP Sgt. Todd Royce.
The injured man was taken back to the hospital, where he died.
Traffic was snarled during the evening commute as the accident and subsequent investigation closed the left southbound lane of the freeway for more than two hours.
UHP troopers say they have no idea why the man was running from the hospital, but the incident is under investigation.