List of primary Interstate Highways

There are 72 primary Interstate Highways in the Interstate Highway System, a network of freeways in the United States. These primary highways are assigned one- or two-digit route numbers. Their associated auxiliary Interstate Highways receive three-digit route numbers. Typically, even-numbered Interstates run east–west, with lower numbers in the south and higher numbers in the north. Odd-numbered Interstates run north–south, with lower numbers in the west and higher numbers in the east.

Route numbers divisible by 5 usually represent major coast-to-coast or border-to-border routes, for example, I-10 connects Santa Monica, California to Jacksonville, Florida, extending between the Pacific and Atlantic oceans. Auxiliary highways have an added digit prefixing the number of the parent highway.

Six route numbers are duplicated in the system. The corresponding highways are in different regions, reducing potential confusion. In addition to primary highways in the contiguous United States, there are signed Interstates in Hawaii and unsigned Interstates in Alaska and Puerto Rico.