I don't mind brainstorming for an answer. I enjoy helping people figure out information about Sonoma County.
Off the top of my head, the only solution I can suggest would be to go to the City of Santa Rosa website and find the traffic counts database there for Hwy 12 near Oakmont. Last time I checked a very thorough traffic count was there,anyway.
If I think of another idea, I'll post it later.
Good luck and call me if you need help with real estate.