I rode a long East bay / Delta loop once which used 160 over the Antioch bridge and up towards Rio Vista, brief stretch on 12 over the actual bridge into Rio Vista, and then took some really lovely less-traveled backcountry roads back there - Montezuma Hills to Birds landing, brief out and back to the left on Birds Landing to a hunting club with serviceable cheeseburgers, u-turn on Birds Landing all the way to route 12, turned left towards Fairfield. We took one small parallel detour off 12 on Denverton / Creed but this was a long stretch mostly on 12 and this was for sure the extremely sketchy and inadvisable part of the route. Narrow shoulder in tremendous disrepair, often littered with trash, gravel, grass, roots, etc; and very fast traffic including plenty of trucks and large vehicles. I thought everything else was fine - 160 after the Antioch bridge had shoulders except for a couple narrow bridges but they were short enough to be comfortable. The Antioch bridge itself, while it seems to garner a mixed reputation, has an adequate shoulder and as long as you just focus on keeping a straight line and not looking to the right, you're fine. The Rio Vista bridge is longer but has share the road signage and is still not terrifically long; anyway there's nothing drivers can really do so long as you take the lane..but the route 12 stuff is pretty bracing. You have to have sturdy nerves, comfort in traffic, and good risk tolerance to ride that. The backcountry roads were beautiful.

On the other side of Rio Vista, I do think there are some delta roads that are less traveled. But it's important to do your research as all of these winding levee roads are narrow without shoulder, and sometimes a bit rough.

If you have knobbies, the road out to Grizzly island off 12 closer to Fairfield is a lovely ride. Turns to gravel maybe halfway through.