Come on out to Yolo County, where our local hills are overpasses!

http://www.solanolinks.com/pdfs/Plan...inks%20Map.pdf

Anything east of Vacaville is going to be flat as a pancake. Quiet flat country roads between Winters, Davis, and Woodland. As recommended above, Pleasants Valley Road is a great ride, but is not "flat" by Yolo County standards.

One other suggestion is the American River Bike Trail that runs from Sacramento to Folsom.

http://www.saccycle.com/bikewaymaps/map6.htm