It is a divided four-lane expressway that serves as an alternate to the nearby Route 29 freeway into Napa from the south.

The northern terminus is an intersection with Route 121, where 121 continues north as Soscol Avenue.

[4] 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.

R reflects a realignment in the route since then, M indicates a second realignment, L refers to an overlap due to a correction or change, and T indicates postmiles classified as temporary (for a full list of prefixes, see California postmile § Official postmile definitions).

[1] Segments that remain unconstructed or have been relinquished to local control may be omitted.