Originally constructed in the early 1920s, it is one of the steepest and most difficult to drive of any California state highway. Until recently it was called the "crookedest road in California".[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]
Major intersections
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. The numbers reset at county lines; the start and end postmiles in each county are given in the county column.
^Federal Highway Administration (March 25, 2015). National Highway System: California (North)(PDF) (Map). Scale not given. Washington, DC: Federal Highway Administration. Retrieved September 17, 2017.
^Natzke, Stefan; Neathery, Mike & Adderly, Kevin (June 20, 2012). "What is the National Highway System?". National Highway System. Washington, DC: Federal Highway Administration. Retrieved July 1, 2012.
^ abCalifornia Department of Transportation. "State Truck Route List". Sacramento: California Department of Transportation. Archived from the original(XLS file) on September 5, 2015. Retrieved June 30, 2015.