 Following the completion of the Mid-County Expressway, in 1996 the Interstate 476 designation was extended to include the entire length of the existing Northeast Extension, replacing PA 9. The Northeast Extension was built between 1955 and 1957 and was originally projected to continue past Clarks Summit to the New York border; however, I-81 was built between these two points instead. Since the extension, I-476 has been the longest auxiliary Interstate Highway in the United States. In fact, it is longer than some mainline interstates such as Interstate 83 in Maryland and Pennsylvania, Interstate 99 in Pennsylvania, Interstate 12 in Louisiana, Interstate 19 in Arizona, Interstate 97 in Maryland, and Interstate 2 in Texas. Additionally, its numbering is unusual since auxiliary interstates beginning in even numbers are typically bypasses or loops rather than spurs. I-476 may be surpassed by Interstate 369 in Texas if I-369 is extended to the Gulf of Mexico.
