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.
Interstate 68
Though the National Freeway was designated as US 48, as the completion of the freeway neared, the possibility of the freeway being designated as an Interstate Highway came up. In the 1980s, the project to improve US 50 between Washington, D.C., and Annapolis to Interstate Highway standards had been assigned the designation of I-68. MDSHA, however, later concluded that adding additional route shields to the US 50 freeway would not be helpful to drivers, since about half the freeway already had two route designations (US 50 and US 301) and drivers on the freeway were already familiar with the US 50 designation. This made the designation to be applied to that freeway more flexible, and so in 1989, the American Association of State Highway and Transportation Officials (AASHTO), the organization composed of the various state departments of transportation that decides route numbering in the United States, approved MDSHA's request to renumber the US 50 freeway from I-68 to I-595. That same year, AASHTO approved changing US 48's designation to I-68. This change took effect upon the completion of the last section of the National Freeway, on August 2, 1991.