UPDATE: 4:25 pm
A fatal accident involving a motorcyclist caused a SigAlert on the northbound I-5 at Balboa Blvd, just south of Santa Clarita for several hours Monday afternoon.
The incident forced the closure of the three northbound I-5 lanes as well as all northbound truck lanes.
By 4:20 p.m., all lanes were re-opened, although traffic continued to suffer.
Aside from the motorcyclist, one other motorist may have been killed in the accident, which involved a big-rig, a motorcycle and a pickup truck.