Interstate 78 (I-78) is an Interstate Highway in the Northeast United States, running 144 miles (231 km) from Interstate 81 northeast of Harrisburg, Pennsylvania, through Allentown, Pennsylvania, and western and northern New Jersey to the Holland Tunnel and Lower Manhattan in New York City.I-78 is a major road linking ports in the New York City and New Jersey area to points west, and sees over 4 million trucks annually, with trucks representing 24% of all traffic.