It's Probably Now Interstate 505

On the 1966 California official highway map, there is an unlabeled route between US 99W at Dunnigan and Interstate 80 on the east side of Vacaville. This route is now Interstate 505. But on this map, the highway clearly is not an Interstate highway, or even a freeway except for a very short segment near Winters. The route could not be US 40 because it went through Sacramento on I-80. There is no indication of how this route could have been numbered.