Note that the route number of an Interstate Highway is even for roads which are primarily east-west routes, and odd for roads which are primarily north-south.

List of one- and two-digit interstate highways in the United States:

Interstate 4Florida
Interstate 5Washington, Oregon, California
Interstate 8Arizona, California
Interstate 10Florida, Alabama, Mississippi, Louisiana, Texas, New Mexico, Arizona, California
Interstate 12Louisiana
Interstate 15Montana, Idaho, Utah, Arizona, Nevada, California
Interstate 16Georgia
Interstate 17Arizona
Interstate 19Arizona
Interstate 20South Carolina, Georgia, Alabama, Mississippi, Louisiana, Texas
Interstate 24Tennessee, Georgia, Tennessee (again), Kentucky, Illinois
Interstate 25Wyoming, Colorado, New Mexico
Interstate 26South Carolina, North Carolina, Tennessee
Interstate 27Texas
Interstate 29North Dakota, South Dakota, Iowa, Missouri
Interstate 30Arkansas, Texas
Interstate 35Minnesota, Iowa, Missouri, Kansas, Oklahoma, Texas
Interstate 37Texas
Interstate 39Wisconsin, Illinois
Interstate 40North Carolina, Tennessee, Arkansas, Oklahoma, Texas, New Mexico, Arizona, California
Interstate 43Wisconsin
Interstate 44Missouri, Oklahoma, Texas
Interstate 45Texas
Interstate 49Louisiana
Interstate 55Illinois, Missouri, Arkansas, Tennessee, Mississippi, Louisiana
Interstate 57Illinois, Missouri
Interstate 59Georgia, Alabama, Mississippi, Louisiana
Interstate 64Virginia, West Virginia, Kentucky, Indiana, Illinois, Missouri
Interstate 65Indiana, Kentucky, Tennessee, Alabama
Interstate 66District of Columbia, Virginia
Interstate 68Maryland, West Virginia
Interstate 69Michigan, Indiana
Interstate 70Maryland, Pennsylvania, West Virginia, Ohio, Indiana, Illinois, Missouri, Kansas, Colorado, Utah
Interstate 71Ohio, Kentucky
Interstate 72Illinois, Missouri
Interstate 73North Carolina
Interstate 74Ohio, Indiana, Illinois, Iowa
Interstate 75Michigan, Ohio, Kentucky, Tennessee, Georgia, Florida (See Also: Alligator Alley)
Interstate 76Eastern: New Jersey, Pennsylvania, Ohio; Western: Nebraska, Colorado
Interstate 77Ohio, West Virginia, Virginia, North Carolina, South Carolina
Interstate 78New York, New Jersey, Pennsylvania
Interstate 79Pennsylvania, West Virginia
Interstate 80New Jersey, Pennsylvania, Ohio, Indiana, Illinois, Iowa, Nebraska, Wyoming, Utah, Nevada, California
Interstate 81New York, Pennsylvania, Maryland, West Virginia, Virginia, Tennessee
Interstate 82Oregon, Washington
Interstate 83Pennsylvania, Maryland
Interstate 84Eastern: Massachusetts, Connecticut, New York, Pennsylvania; Western: Utah, Idaho, Oregon
Interstate 85Virginia, North Carolina, South Carolina, Georgia, Alabama
Interstate 86Eastern: New York, Pennsylvania; Western: Idaho
Interstate 87New York
Interstate 88Eastern: New York; Western: Illinois
Interstate 89Vermont, New Hampshire
Interstate 90Massachusetts, New York, Pennsylvania, Ohio, Indiana, Illinois, Wisconsin, Minnesota, South Dakota, Wyoming, Montana, Idaho, Washington
Interstate 91Vermont, Connecticut, Massachusetts
Interstate 93Vermont, New Hampshire, Massachusetts
Interstate 94Michigan, Indiana, Illinois, Wisconsin, Minnesota, North Dakota, Montana
Interstate 95Maine, New Hampshire, Massachusetts, Rhode Island, Connecticut, New York, New Jersey, Pennsylvania, Delaware, Maryland, Virginia, North Carolina, South Carolina, Georgia, Florida
Interstate 96Michigan
Interstate 97Maryland
Interstate 99Pennsylvania
Interstate 238 California (an unusual exception to the rules)

Hawaii interstates include:

Note: Three-digit interstates (with the exception of I-238) are to be discussed in the same article as the one- or two-digit interstate that the three-digit interstate is a spur or loop of, even if plans to build the connection between the "spur" and the main highway never materialized. For example, I-170 is discussed in the I-70 article, and I-278 in New York and New Jersey is discussed in the I-78 article, even though no connection exists between I-78 and any of its spurs or loops. I-238 is an exception because it is not a loop or spur of any one- or two-digit interstate (no I-38 exists anywhere near I-238).