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The Road to Hana in Maui takes about three hours to cross the island; and traffic can sometimes be horrible. If my conscience ever evaporates, I plan to build a six-lane toll freeway (I-H4) to cut the trip time to 30 minutes. This should enhance economic development in the Hana metro area and Maui in general.
Frequent drivers who get their ticket stamped 10 times will get a free H4 T-shirt with a gecko (this is Maui's largest industry).
I-H-201 is Hawaii's only three-digit interstate, and the nation's only four-character interstate. (In the past, Idaho's I-184 was known briefly as I-180N.) Until July 2004, it was also a "secret" interstate, signposted only as state route 78.
How H-201 came about
H-201 is unique not only in name but in history: its designation arose from a Hawaii DOT (HDOT) intent to correct a "nonconforming condition" at the eastern H-1 / HI 78 junction. At this point, three lanes of mainline road serve HI 78, which should be a right-hand exit; and two lanes of right-hand exit serve H-1, which should be the mainline. (See diagram and details by Oscar Voss.) Motorists not expecting this could miss the turn and no longer be on an interstate highway. HDOT's plan was to designate HI 78 as an interstate.
You might ask why the interchange was built this way. The answer: the airport segment of H-1 was the last to open, and was connected via ramps to the existing H-1 / HI 78 mainline. Here are the approximate opening dates of each freeway leg leading from the interchange:
The addition of HI 78 to the interstate highway system was approved by the FHWA on Nov. 1, 1989.  Before we get to the number selection, which occurred later, consider two things:
We need a number; or, "H-1B" has political overtones
So... how would you number a loop of Interstate H-1? (By the way, Hawaii interstate numbers officially have a hyphen after the "H", even if some signs omit it. )
On June 2, 1990, HDOT requested the number "I-H-1A," to which AASHTO's response was "Action Withheld." (Interstates with letter suffixes, fairly common 30 years ago, are discouraged today.)
On December 7, 1990, HDOT went forward with plan B: proposing "I-H-101." AASHTO thought better of this suggestion, but because the road met H-1 at both ends, it approved the highway instead as I-H-201. 
What about the "secret" designation mentioned earlier?
Not only did the new H-201 number neither fix the interchange nor help in funding, for over a decade HDOT decided it was better not to signpost it. Three reasons were cited for this:
C. C. Slater wrote: "I don't like the [renumbering], logical though it is. Heck, I grew up riding up and down that road -- seeing the big elevated junction at Kam Highway for the first time was what got me started in roads anyway. The [SR 78] designation has sort of a sentimental value!" 
In mid-2004, no longer secret
Why was H-201 signed at this point, after 14 years? HDOT spokesman Scott Ishikawa (no relation) told the Honolulu Star-Bulletin that since they were erecting new signs, they were required to use the new H-201 designation instead of Route 78. . (However, there are at this writing 13 unsigned 3di's.)
Oh well. After a while it just seems really mainland to worry about such things.
State highway H201? No, that's an error
Some 2004 atlases show H-201 as state route H201, which is incorrect; the route's designation hasn't changed. A typo in the Hawaii DOT's website, which uses the state designation, could have misled some mapmakers.