Sadly, there's no really great way to get from Napa to Martinez. This would be my recommendation for the least trafficky route: http://ridewithgps.com/routes/1216392
. There's no getting around riding on Hwy 29 for some of it -- very trafficky, but fairly wide shoulder. I've routed you off of 29 where possible. The bike path along the south side of Hwy 37, in Vallejo is a bit tricky to find, but it's there. The Scenic Drive portion along the south side of Carquinez Strait is beautiful, though hilly. There's about a 100 foot section of single track there, and some of the pavement is iffy, but most of it is closed to cars. This is the way I usually go to Martinez, except I usually stay on Hwy 29 through American Canyon. I would do it a little differently coming from Martinez to Napa, so if that's the direction you're going, let me know.