For many years the route was hidden, but was signed in 2005 in anticipation of a complete redesign - reconstruction of Weed Boulevard.

[citation needed] Previously, the only visual proof of the route was the bridge ID that was spraypainted on the base of the Interstate 5 overpass, i.e. "265 overcrossing".

[3] Except where prefixed with a letter, postmiles were measured on the road as it was in 1964, based on the alignment of U.S. Route 99 as it existed at that 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.