Officials have issued evacuation orders, but did not immediately say how many people are impacted. The fire is in a rural area with scattered homes.
Thousands of people were stuck after it ignited Wednesday just before 1 p.m. There were abandoned trucks destroyed by the flames as the fire jumped I-5.
"It felt like it was coming to us so some of the guys that were in front of us, I don't know like five or ten trucks in front of us were closer to it, they had to drop their trailers and turn around," said Julio Vega, motorist.
VIDEO: Close encounter with wildfire burning next to I-5 near Redding caught on camera
The U.S. Forest Service has launched a full aircraft attack and ground response.
Officials say the cause of the fire was human, but they did not go into specifics.