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For about two years, until Jan. 2001, this page used to be in French, merely to mock Canadian Content and Quebec language laws. Nothing is funny forever, and my bad French was preventing many people from deciphering the page. The page is now Yankified, and will remain in English from now on.
Overdrive Magazine editor G.C. Skipper had this to say in 1995 about Keystone roads: "Preachers say, 'The road to hell is paved with good intentions.' We'd like to add, 'Amen brother, and it runs through Pennsylvania.'" For many years, PA roads were recognized by truckers as the worst in the United States; in 1999, the state was unseated by Arkansas for this dishonor.
I-179 (numbered as another interstate) Pennsylvania
Interstate 179 was a preliminary numbering for plain old I-79 north of I-90 toward Erie. It was approved by AASHO on Nov. 12, 1958.  When I-179 was completed there 10 years later, it was signed as part of I-79.
The idea of using a 3-digit number for the last bit of 2-digit interstate ending in a city is not common practice, but might have been in vogue when the system was developed. In 2003, Tennessee proposed the same idea when I-26 is extended to I-81: the remaining portion of I-181 leading to Kingsport would become I-126.
I-179 is one of our more obscure former 3di's. For several years, all we had seen was a Rand McNally map and a few DOT documents, and it wasn't certain whether I-179 was part of I-79 or a cancelled parallel route.   Finding the AASHO document seems to have solved this riddle.
19.52 miles ; from I-79 into Pittsburgh, and back to I-79 again. The 13.5-mile, $550 million "Parkway North" connection from Pittsburgh to the northwest was fully open on Sept. 16, 1989. This section features reversible HOV lanes. 
Originally I-79 was to have taken Interstate 279's alignment, going into Pittsburgh, while I-279 would follow I-79's alignment west of the city. In October 1972, both I-79 and I-76 were relocated away from the city center, leading to I-279's current route and a new designation (I-376) for the highway entering from the east. 
See also: Pennsylvania Highways: Interstate 279 (Jeff Kitsko)
I-479 (numbered as another interstate) Pennsylvania
Old numbering for what became I-579 in Pittsburgh; number was changed in 1971. 
See also: Pennsylvania Highways: Interstate 479 (Jeff Kitsko)
1.57 miles ; from I-279 to a point near I-376 in downtown Pittsburgh. PennDOT maps show this route partly constructed in 1967, and called I-479. In 1971, the number was changed to I-876 (when Parkway East was called I-76 instead of I-376, and the now-76 portion of the Pennsylvania Turnpike was called I-80S). In 1972, 479 became 579, and that part of 76 became 376.
See also: Pennsylvania Highways: Interstate 579 (Jeff Kitsko)
For grins, here's the old French description for Interstate 579:
I-579 Pennsylvanie1,57 milles; à Pittsburgh, passant au sud d'I-279 devant l'arène civique, où les Pingouins loathsome jouent. Autour 1990 réalisé.
Les cartes de PennDot montrent cette route en partie construite en 1967, et ont appelé I-479. En 1971, elle a été changée en I-876, comme I-76 toujours terminé à la Triangle d'Or moment-là (le reste du turnpike de PA en l'Ohio s'est appelé I-80S). En 1972, quand 376 ont obtenu son nombre courant, a ainsi fait I-579.