Pennsylvania Route 378
Length: About 2.3 miles
Termini: US 22 (Lehigh Valley Thruway) east of the Airport Interchange and Spring Street in downtown Bethlehem.
Reason for decomissioning: Interstate 78 was rerouted to a southerly bypass of Allentown-Bethlehem-Easton because of Pennsylvania's slowness to upgrade the Lehigh Valley Thruway (US 22) from its own "standards" and to bypass Easton with a limited access connector between the Thruway and the 78 routing. (Currently, there is a project to "upgrade 22", according to PennDOT, but it's basically just lengthening offramps. The route itself is still Pennsylvania standard.)
History: 378 was to have its own 1xx twin (like 175 and 375 in St. Petersburg, Florida) in Allentown, I-178. This was canceled because the Liberty Bell Shrine was in the way, and with some NIMBY opposition to losing 6th and 7th Streets.
378 is the only of the two x78 spurs in PA to ever have been built (the 178 story is in the previous paragraph). I-78 originally was to be routed on the Lehigh Valley Thruway (US 22), but as we all know, the Thruway will never be upgraded to decency. So I-78 was routed on the new bypass (which was finished after a burst of Pennsylvanian funding in the 1980s) and US 22 was kept on the Thruway.
Because of this, I-378 had no reason to live, literally. Since it missed its parent route by 2 miles due to the rerouting, it had to be decomissioned and sent to the state route dump. However, as a compensation for this orphaned route, PA 378 also was extended as a state highway to PA 309.