The man was driving his Dodge Ram pickup truck north on Highway 101 near the Atherton Avenue on-ramp in Novato when for unknown reasons, he went off the roadway and slammed into the back of a parked tank trailer, Officer Andrew Barclay said.
Emergency responders attended to the pickup truck driver, who was pronounced dead at the scene.
The driver of the big rig had pulled over onto the side of the roadway and was sleeping in the rig's sleeper berth when the crash occurred and was uninjured.
Barclay said the pickup truck driver's name is being withheld as officers attempt to contact the man's family.
CHP investigators do not believe alcohol was a factor in the crash.
Since the crash occurred on a wide shoulder off of the roadway, there was only a minimal effect on traffic.