Part 1; the history of Hawaii Route 380
The Island of Maui seemingly was not part of the original World War II era Hawaii Route System. Circa 1955 the United States Bureau of Public Roads renumbered the Hawaii Route System. The 1955 Hawaii Route Renumbering saw most of the conventions utilized by the current Hawaii State Route System established. Primary Hawaii Routes were given two-digit numbers whereas Secondary Hawaii Routes were given three-digit numbers. The Hawaii Routes were assigned in sequence for what Island/County they were located on coupled with what Federal Aid Program number they were tied to. In the case of the Island of Maui it was assigned numbers in the range of 30-40.
Hawaii Route 38 was assigned to a small corridor which connected Hawaii Route 36/Hana Highway near Kahului Airport southwest to Hawaii Route 35/Puunene Avenue via the publicly accessible portion of Dairy Road. The earlier iteration of Hawaii Route 38 along Dairy Road can be seen on the 1955 United States Geological Map of Wailuku.
According to hawaiihighways.com an easement was granted to the State of Hawaii during 1971 to construct a State Highway on what was the privately owned part of Dairy Road. Dairy Road west of Puunene Avenue was reconstructed to four-lane expressway standards as Hawaii Route 380 which connected to new terminus at Hawaii Route 30/Honoapiilani Highway. Hawaii Route 380 can be seen first time on the 1983 United States Geological Survey Maps of Maui, Wailuku and Maalaea. Hawaii Route 380 is displayed as following Keolani Place, Dairy Road and Kuihelani Highway from Kahului Airport west to Hawaii Route 30.
Modern maps still display Hawaii Route 380 following Keolani Place to Kahului Airport despite it being field signed as Hawaii Route 36A. It is unclear when signage on Keolani Place was swapped to Hawaii Route 36A.