A crash between a big rig and a Ford Mustang shut down all but one lane of the northbound 5 Freeway in Newhall on Monday evening.
The collision was first reported about 7:20 p.m. on the freeway near Calgrove Boulevard, according to the California Highway Patrol.
The big rig jack-knifed and overturned, forcing the closure of the four left-hand lanes of northbound traffic for an "unknown duration," CHP logs show. Only the right-hand-lane remained available.
It was not immediately clear whether the crash involved injuries.
A witness reported seeing the Mustang traveling at high speed along the right shoulder before crossing multiple traffic lanes and colliding with the big rig, CHP logs indicate.
Officials estimated it would take several hours to clear the wreck, with the lanes remaining blocked through at least midnight.