A 54-year-old Sebastopol man was killed in a car crash on Todd Road south of Santa Rosa on Wednesday night, the CHP said.
His name was being withheld Thursday, pending notification of his family, the Sonoma County Coroner's Office said.
The man died after drifting off the road in the dark and rain and crashing into a ditch near Primrose Avenue, where his 1991 Acura Integra struck a concrete culvert abutment head-on, authorities said.
He was not wearing a seatbelt, which contributed to the severity of his injuries, the CHP said.
It was unclear what caused the car to run off the road, authorities said.