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Despite being designated an Interstate in 1989, until recently the route was a "hidden" Interstate signed only as State Route 78. Interstate signs started appearing in mid-2004. (The section of the Moanalua Freeway between Kamehameha Highway (state route 99) and Interstate H-1 remains designated solely as route 78.) See Notes below for further explanation.
| Miles | km | ||
| 4 | 6 | Hawaii | |
| 4 | 6 | Total | |
The Federal Highway Administration approved the addition of Interstate H-201 to the Interstate Highway System on November 1, 1989. The Hawaii Department of Transportation asked that the Moanalua Freeway be reclassified as an Interstate so that the interchange with H-1 at the eastern end could conform to federal highway standards.
At first, the highway was initially designated H-1A, but federal highway policy does not allow alphabetic suffixes in Interstate numbers. The final designation, H-201, conforms to the general rules for three-digit Interstate "loop" routes.
Until 2004, the state Department of Transportation chose not to sign H-201 as such, instead retaining the designation Hawaii route 78. Reasons given include:
In July 2004, in conjunction with a major resurfacing of both sides of the freeway, it was decided to bring the signage in line with the official designation.