Interstate 78 in Pennsylvania

Interstate 78 (I-78) is a major east–west Interstate Highway stretching from Union Township, Lebanon County, Pennsylvania, in the west to the Holland Tunnel and New York City in the east. In Pennsylvania, I-78 runs for about 78 miles (126 km) from the western terminus at I-81 in Union Township east to the New Jersey state line near Easton in Northampton County.

Much of I-78 west of Allentown predates the Interstate Highway System as a freeway upgrade of U.S. Route 22 (US 22), which runs concurrently with I-78 between Bethel Township and Upper Macungie Township. The Interstate, originally planned as Interstate 80N (I-80N), was supposed to continue its concurrency with US 22 to the New Jersey state line but was realigned to the south due to local opposition.

Two auxiliary routes, Interstate 178 (I-178) and I-378, were also planned as spurs into Allentown and Bethlehem respectively, but I-178 was never built and I-378 became Pennsylvania Route 378 (PA 378) since it did not connect to I-78.