East of Suisun it becomes two lanes again and crosses south of Travis Air Force Base, through rolling fields with numerous wind turbines.

At Rio Vista, SR 12 crosses the Helen Madere Memorial Bridge over the Sacramento River, entering the California Delta.

[4][5] This stretch of the river has twice been the site of much-publicized inland excursions by humpback whales, presumably veering off course while making their annual migrations along the Pacific coast.

In 2007 a humpback mother and calf dubbed Delta and Dawn circled for several days just upstream of the bridge, seemingly reluctant to pass under it again after having spent the previous three or four weeks upriver near Sacramento.

[11] SR 12 has been designated by various state laws and through local usage as the Valley of the Moon Scenic Route (the portion running through Sonoma County, though the first sign with this name is at the intersection of Farmers Lane and Fourth Street in Santa Rosa);[12] Luther Burbank Memorial Highway from Santa Rosa to Sebastopol (after botanist Luther Burbank).

[20] Plans also called for Route 12 to be built as a freeway from Farmers Lane to Melita Road, in Santa Rosa; however, as of 2014, this project was delayed due to community opposition.

[21] Except where prefixed with a letter, postmiles were measured on the road as it was in 1964, based on the alignment that existed at the time, and do not necessarily reflect current mileage.