The route begins at
SR 37 in
Sears Point, and then runs past the
Tolay Lake basin and across
Tolay Creek near
Infineon Raceway. As it continues northward through
Sonoma County, it meets
SR 116, where it then veers east. SR 121 then enters
Schellville, where it begins a short
overlap with
SR 12. Upon leaving, it begins another overlap with
SR 29, which happens to be a freeway, in
Napa County. When it leaves, it continues northward and meets
SR 221 in
Napa. As it leaves the city, it continues northward for several miles before reaching its north end at
SR 128 near
Lake Berryessa. SR 121 is part of the
California Freeway and Expressway System, but is not part of the
National Highway System, a network of highways that are considered essential to the country's economy, defense, and mobility by the
Federal Highway Administration. SR 121 is eligible to be included in the
State Scenic Highway System, but it is not officially designated as a scenic highway by the
California Department of Transportation. The
California Legislature named the route
Carneros Highway from its southern terminus to its junction with SR 29 in
Napa, after the Carneros settlement. The stretch in Sonoma County between SR 37 and SR 116 runs along
Arnold Drive, and along
Fremont Drive between SR 116 and the Napa County line. ==History==