List of Interstate Highways
There are 70 primary Interstate Highways in the Interstate Highway System, a network of controlled-access freeways in the United States. They are assigned one- or two-digit route numbers, whereas their associated "auxiliary" Interstate Highways receive three-digit route numbers. Typically, odd-numbered Interstates run south-north, with lower numbers in the west and higher numbers in the east; even-numbered Interstates run west-east, with lower numbers in the south and higher numbers in the north. Highways whose route numbers are divisible by "5" usually represent major coast-to-coast or border-to-border routes (ex. I-10 travels from Santa Monica, California, to Jacksonville, Florida, traveling from the Pacific to Atlantic oceans). Additionally, auxiliary highways have their numbering system where a different number prefixes the number of its parent highway.
List of Interstate Highways in Pennsylvania
The list of Interstate Highways in Pennsylvania encompasses 23 Interstate Highways—12 primary routes and 11 auxiliary routes—which exist entirely or partially in the U.S. state of Pennsylvania. In Pennsylvania, most of the Interstate highways are maintained by the Pennsylvania Department of Transportation (PennDOT). Some stretches are also maintained by the Pennsylvania Turnpike Commission, Delaware River Port Authority, the Delaware River Joint Toll Bridge Commission, and 2 short stretches maintained by the New York State Department of Transportation (these being the Delaware River bridge on Interstate 84 and a short stretch of the future Interstate 86 in Bradford County). Interstate highways make up three percent of all roadway lane miles in Pennsylvania and have a combined length of within the state. Twenty-four percent of all vehicle traffic is on the Interstate system.