An overturned big rig is blocking lanes on southbound Interstate 5 near Laguna Boulevard.
The crash happened just before 2 p.m.
The CHP says witnesses described the truck drifting into the center lane from the slow lane, and then over correcting. The truck lost control and hit a guard rail.
The driver of the truck was taken to the hospital.
One lane is open by the accident, crippling the flow of traffic. The scene should be clear by 5 p.m.