Sgt. Dave Pedersen said the 19-year-old woman said she was driving on Bennett Valley Road in Rincon Valley around 10:30 p.m. when the driver of a vehicle behind her began flashing their headlights.
At one point the vehicle pulled beside her in the opposing traffic lane and appeared to try to get her to pull over, Pedersen said.
The victim pulled over on Bennett Valley Road southeast of Enterprise Road. A man with the neck of his shirt pulled over his nose got out of his car and approached the woman, Pedersen said. The man said there had been a motorcycle accident and he asked the woman to call 911, Pedersen said.
As the woman was making the phone call, the man told her to hang up or he would harm her. The man then committed sexual battery on the woman, walked back to his red or maroon four-door sedan and left in an unknown direction, Pedersen said.
The suspect, who is in his late 20s, was wearing a turtleneck sweater, Pedersen said.
Anyone with information is asked to call the sheriff's department at (707) 565-2650.