A semitrailer traveling north on Interstate 15 in South Salt Lake caught fire at about 4:20 p.m. Saturday, sending thick smoke over the freeway and forcing the closure of every northbound lane except the high-occupancy-vehicle lane, authorities said. Utah Highway Patrol spokesperson Cpl. Eddie Wright said the driver escaped without injury. Firefighters battled flames that reportedly involved propane tanks and spread to roadside brush, while a Chinook helicopter was observed heading toward the area as part of the response. By 4:45 p.m., the Utah Department of Transportation had reopened the HOV lane to limited traffic, but warned motorists to expect prolonged delays and consider alternate routes. Investigators are examining a potential mechanical failure as the cause of the blaze, which did not involve other vehicles.