Interstate 378 was the original designation for the freeway connecting U.S. 22 (Lehigh Valley Thruway) near Lehigh Valley International Airport (ABE) with Downtown Bethlehem. Designated Pennsylvania State Route 378, the short spur winds southeast from the Westgate and Notre Dame Hills neighborhoods to the Hill to Hill Bridge spanning the Lehigh River. Interchanges along the route are numbered sequentially and increase from north to south.
The Bethlehem freeway spur was constructed between 1966 and 1968.1 The route was signed as I-378 until the proposed realignment of Interstate 78 south of Allentown and Bethlehem was finalized. The American Association of State Highway Officials (AASHO) approved the elimination of Interstate 378 and the adjacent Allentown Spur (I-178), on June 30, 1970. I-78 was subsquently dropped from the Lehigh Valley Thruway, which remains as U.S. 22. The former I-378 was renumbered as part of Pennsylvania 378, a 9.55-mile long route stretching south to Pennsylvania 309 in Upper Saucon Township.
Interstate 378 was also previously proposed to extend south across the Lehigh River to an interchange with a separate planned freeway: Constitution Drive to the west and South Side Spur to the east. The South Side Spur was projected to go east and south to the realignment of I-78.