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.
National Scenic Byway
The most-scenic byways are designated All-American Roads, which must meet two out of the six intrinsic qualities. The designation means they have features that do not exist elsewhere in the United States and are unique and important enough to be tourist destinations unto themselves. As of November 2010, there are 120 National Scenic Byways and 31 All-American Roads, located in 46 states (all except Hawaii, Nebraska, Rhode Island, and Texas).