A semi-truck carrying liquefied petroleum gas drifted into the median, landed in a ditch and overturned, spilling the gas.
Caltrans said the freeway is expected to be shut down until at least noon between Hector and Ludlow roads.
The route is heavily traveled by semi-trucks. Area residents reported smelling the spilled gas, but no evacuations were ordered.
Hazmat crews were on the scene.