The fire was reported on the north side of the freeway, between Las Virgenes and Lost Hills roads, at 2:52 p.m.
By 3:25 p.m., it had burned up a hillside into grassy wildlands.
All five lanes of the westbound freeway -- also known as U.S. 101 north - - were blocked. Traffic quickly backed up into Calabasas, where the freeway was closed by the CHP.
A massive traffic jam ensued, as there are no major alternative routes from the San Fernando Valley to the Thousand Oaks area.
At about 3:30, the CHP reopened just one westbound lane, and they issued a SigAlert as they predicted the other three lanes would be closed for several hours as firefighters worked on the burning hillside.