State troopers responded to the scene, on southbound I-5 at the Interstate 405 exit ramp, at about 2:15 a.m. Thursday after receiving a report of a serious accident.
The driver, a man in his 40s, was found dead at the scene. His name was not released pending notification of next of kin.
A preliminary investigation found that the man was driving south on I-5 in a red 2002 Ford Ranger pickup when he drifted off the roadway to the right and onto the grass shoulder of the off-ramp to southbound I-405.
The pickup slammed into a signpost, then rolled several times down an embankment before landing in the ditch.
The driver was not wearing a seat belt and was partially ejected from the vehicle. There were no passengers in the pickup and no other vehicles were involved in the accident.
It is unknown whether drugs or alcohol were a factor in the crash.