A three-car crash shut down Highway 101 northbound late Tuesday night, according to the California Highway Patrol.
The collision occurred around midnight at Highway 101 near Highway 116 in Cotati, a CHP officer said.
Just after the accident, and before emergency crews arrived, the left and part of the middle lanes were blocked with damaged vehicles and a field of debris, said James Long of Santa Rosa, a witness who had been driving home from work.
It was very dark, but someone had laid out flares and was waving a bright flashlight from the shoulder to warn oncoming drivers, the witness said.
"Things can happen very quickly," Long said. "This could have been a lot worse if it wasn't for that quick-thinking person."
There were no major injuries, and drugs or alcohol werte not suspected as a cause in the crash, a CHP dispatcher said.