What killer was this?

Hello,

My mother said she was hitchhiking in northern California in the mid-80s. She said that a man picked her up and continued driving when she requested/demanded he stop. He drove toward a rural area and stopped on a gravel road. He then presented a knife and threatened my mother, she managed to escape.

She's obviously very traumatized about this, and I am curious if this person was identified? Any theories?