Highway 12 runs East to West from it's intersection with Highway 49 in San Andreas to it's intersection with Highway 29 in the Napa Valley. Locally it intersects with Highway 113 between Rio Vista and TAFB. After Fairfield it joins Interstate 80 until, shortly after the 80/680 interchange, where breaks off towards Napa. In Napa it splits off with 29 at 29 & 221 then dissapears in the confusing abyss that is Napa.

It's a two-lane highway characterized by moderately sharp hillside turns. Expect the drive to go slowly if there are trucks on the road, faster in daylight. There are no stops after Interstate 80 until you get close to Highway 29. At that point, you will encounter some stoplights. If you should run out of gas on that part Highway 12, you are sadly out of luck. Highway 12, west of I-80 runs south under the air base, the planes are so low they can light up the inside of your car at night, a worthy stopping point to watch planes take off at night.