Carolyn Dusharme, 81, was pronounced dead on the scene in a field near Fifth Street West and Leveroni.
She was likely unconscious or suffering a medical emergency at the time of the 9:50 a.m. accident, said CHP Officer Jaret Paulson.
There was no sign of the 2001 Mitsubishi Gallant slowing or braking as it traveled southbound on Fifth Street West.
Paulsen said the vehicle then drove through the guardrail at the Leveroni intersection, continued through a field and crashed into the far embankment of Sonoma Creek. The car flipped on its side after impact.
“It looks like she wasn’t conscious or coherent at the time,” Paulson said.