Interstate 476 (I-476) is a 132.1-mile (212.59 km) auxiliary Interstate Highway of Interstate 76 in the U.S. state of Pennsylvania designated between Interstate 95 near Chester and Interstate 81 near Scranton, serving as the primary north–south Interstate corridor through eastern Pennsylvania. It consists of both the 20-mile (32.19 km) Mid-County Expressway, locally referred to as the "Blue Route" (although no signs exist with that designation), through the suburban Philadelphia-area counties of Delaware and Montgomery, and the tolled, 110.6-mile (177.99 km) Northeast Extension of the Pennsylvania Turnpike connecting the Philadelphia metropolitan area with the Lehigh Valley, the Poconos, and the Wyoming Valley. The Blue Route passes through suburban areas, while the Northeast Extension predominantly runs through rural areas of mountains, forest and farmland, with development closer to Philadelphia and in the Lehigh Valley and the Wyoming Valley. I-476 intersects many major roads including Interstate 76 (the Schuylkill Expressway) in West Conshohocken, Interstate 276 (the Pennsylvania Turnpike) in Plymouth Meeting, U.S. Route 22 near Allentown, and Interstate 80 near Hickory Run State Park.
At its opening in the early 1970s, I-476 was a four-mile-long, four-lane spur expressway connecting the Schuylkill Expressway with the Pennsylvania Turnpike in Plymouth Meeting. It significantly helped reduce congestion through King of Prussia and the Schuylkill Expressway as it provided a more direct link from the Main Line suburbs to the Northern suburbs and New Jersey. The highway was initially planned to connect down to I-95 in Delaware County, but due to environmental and local opposition, this portion did not open until 1990.
In 1996, the I-476 designation was affixed to the pre-existing Northeast Extension of the Pennsylvania Turnpike, replacing Pennsylvania Route 9. This was an older four lane pre-Interstate limited access highway. Of earlier design, its cross section was very narrow, with only 10 feet between opposing lanes of traffic in places. This extended I-476 north of Plymouth Meeting to Clarks Summit (near Scranton) as a part of the Pennsylvania Turnpike system. The crown jewel of this expressway was a state-of-the-art three-level interchange, to connect the new I-476 with the older Pennsylvania Turnpike and the Northeast Extension. This provided direct access to both I-276 east and I-476 north, now on the Northeast Extension. The Northeast Extension served the Lehigh Valley at U.S. Route 22, with connection to I-78, the southern bypass of Allentown/Bethlehem. With the completion of the Northeast Extension, I-476 surpassed I-495 in Massachusetts as the longest auxiliary Interstate Highway, a record it will hold until the completion of I-369 in Texas. I-476 was widened to 6 lanes between I-76 and I-276 in 1990-1992. It was later widened to 6 lanes up to Lansdale between 2011-2017.