A man sustained major injuries after his car slammed head on into a big-rig on Highway 84 in Sunol on Wednesday morning, authorities said.

The truck-tractor combination caught fire as a result of the crash, spreading to roadside brush and causing an extensive cleanup effort that closed the commuter highway down for nearly six hours.

The incident occurred just before 9:40 a.m. Wednesday when a Honda sedan traveling on Highway 84 near Little Valley Road apparently drifted into the oncoming lane and collided with a truck-tractor combination big-rig, according to Officer Tyler Hahn of the California Highway Patrol.

Officers are still working to determine why the Honda entered the other lane, but they couldn't obtain an in-depth statement from the Honda driver due to the severity of his injuries, Hahn said. The Honda driver, whose name was not released, was airlifted by CHP helicopter to Eden Medical Center in Castro Valley for treatment.

The big-rig driver sustained minor injuries, according to Hahn.

The collision flung both vehicles off the roadway in opposite directions and the big-rig caught fire. Vegetation on the side of the highway caught fire around the truck, but firefighters were able to quickly contain the flames. Cal Fire, Alameda County Fire and Livermore-Pleasanton Fire departments were involved in the response.

Highway 84 was closed in both directions until approximately 3:30 p.m. Wednesday due to the difficulty in removing the badly burned truck and tractor from the scene, according to Hahn.