California Highway Patrol press release: 

On March 29, 2016, at approximately 3:00 AM, A 2016 Hyundai sedan driven by a male, whose name is currently being withheld pending notification of next of kin, was traveling northbound on US-101, north of the Laytonville-Sherwood Road.  For reasons still under investigation, the driver allowed the sedan to drift onto the right shoulder.  The vehicle then briefly collided with the east guardrail before descending an embankment where it overturned onto its roof and collided with a tree.  The driver, and only occupant of the vehicle, sustained fatal injuries as a result of the collision and was pronounced deceased at the scene.

Agencies on scene were Cal-Trans, Cal-Fire, Laytonville Volunteer Fire, Mendocino County Sheriff, and CHP.

This collision remains under investigation by the California Highway Patrol.  Whether or not drugs or alcohol were a factor in this collision is also being investigated.