CA 12 corridor improvements

[skluth]

Starting a new thread before the CA 37 topic gets derailed.

Definitely agree this will be a huge undertaking.  Politics aside, it would be interesting to see what the costs of an inland (121) alignment could be.  While a raised structure for the entire segment would not be necessary, there would be substantial right of way acquisition costs; the current 12/29 bridge over the Napa River would likely need to be widened to at least six lanes as would Jameson Canyon/12; at least three new major interchanges would be needed (12/29/121, 29/12/221. 29/12/Jameson Cyn); several new smaller interchanges including @116, @12/Broadway to Sonoma, Napa Rd, Old Sonoma Rd; plus a viaduct or substantial berm in the Schellville area since that already floods regularly; and yet another redesign of the 12/80/680 entanglement in Fairfield.  Open checkbook, start writing . . .

The 680/12/80 interchange redesign is already in progress though construction hasn't started (essentially the 680 mainline will be realigned away in its last mile from the direct north-south trajectory to feed right into the 12 west mainline going towards Napa)
https://sta.ca.gov/project/i-80-i-680-sr-12-interchange/

There is also a proposed diamond-with-roundabout junction for 29/12 and 221:
https://napavalleyregister.com/news/local/caltrans-wants-napa-feedback-for-roundabout-design-for-highway-at/article_d9cb62e5-5513-5c7b-afba-1a2ee7bbdd19.html
https://napavalleyregister.com/news/local/napa-county-makes-a-transportation-wish-list-for-the-decades/article_4fc98b6b-b25f-5cfb-b11e-98d78c57d476.html

I wish the website for the 680/12/80 interchange was better. I'd really like to see the final plans, but can't figure out a link from that site.

IIRC, there originally was just a roundabout proposed for 29/12 and 221 with a lot of negative comments on how this would be a disaster. I'm glad to see this idea which I think would probably work a lot better for traffic flow. I know a lot of people here hate two-lane roundabouts and this does have two lanes on some parts. But the roundabout is big enough to allow good traffic throughput and it does get the roundabout off the main highway.