"You could hear the explosions from people`s vehicle tires popping from the heat," one driver, Lance Andrade, said.
There have been no confirmed reports of injuries.
Footage from news helicopters shows dramatic pictures of people walking away from the flames, covering their faces and trying not to breathe in the noxious smoke and fumes.
Several lanes of the road were closed, causing traffic to back up for kilometres.
"It`s crazy, you`re watching black clouds and white clouds of smoke, there`s a ridgeline off to my right... and it looks like any second flames will come over the ridgeline,`` said 43-year-old Chris Patterson, quoted by AP news agency.
Several dozen vehicles have been abandoned - and some are completely engulfed in flames.
Firefighters asked evacuating motorists to leave their keys in the cars - in the hope that they could clear the cars once the fire is under control.
Aircraft are dropping flame retardant chemicals on the fire.
The I-15 is the main road linking Los Angeles and Nevada, and is particularly busy at the weekends as southern Californians head to Las Vegas.
Wildfires are common in California, but have been exacerbated by the four-year drought gripping the state.
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