[4] Originally constructed in the early 1920s, it is one of the steepest and most difficult to drive of any California state highway.

[citation needed] Vehicles more than 39 feet (12 m) in length are banned from the Hopland Grade, due to its many tight hairpin turns and curves.

[2] 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).

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