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 Highways in Alaska
Interstates in Alaska follow the numbering system Interstate A-n, where n represents the number of the Interstate. This follows the similar numbering systems for Hawaii and Puerto Rico. The Interstate Highway System was expanded to Alaska in 1976, by the Federal-Aid Highway Act of 1976, which defined the system for Interstates in Alaska and Puerto Rico under Title 23, Chapter 1, Section 103 (c)(1)(B)(ii) of the U.S. Code.