Interstate 78 travels 75 miles east from I-81 in Lebanon County to Allentown, Bethlehem and Easton as part of a 143-mile route joining the Harrisburg area with Newark, NJ and New York City. The Interstate replaced four lane highway sections of U.S. 22, including the Lehigh Valley Thruway through Allentown-Bethlehem for a period of time. U.S. 22 remains overlapped with the freeway from Hamlin (Exit 8) east to Chapman (Exit 51).
A new alignment for Interstate 78 south of the Allentown-Bethlehem area was proposed by the early 1980s. When it opened on November 21, 1989,1 I-78 was fully complete in the Keystone State. The realignment of the freeway mainline away from the Lehigh Valley Thruway marooned Interstate 378, a freeway spur into downtown Bethlehem, leaving it redesignated as PA 378.