On June 29, 1956, President Dwight Eisenhower signed the Federal-Aid Highway Act of 1956, which created a 41,000-mile “National System of Interstate and Defense Highways” that would become America’s most important transportation infrastructure—and one of the world’s modern wonders.

Designed to reach every city with a population of more than 100,000, the system significantly changed American landscapes and lives, increasing ease of travel and affecting the way business is conducted.

Major Interstate routes are designated by one- or two-digit numbers. Routes with odd numbers run north and south, while even numbered run east and west. All but four state capitals are directly served by the Interstate System. Those not directly served are Juneau, Alaska; Dover, Del.; Jefferson City, Mo.; and Pierre, S.D.

The oldest Interstate segments actually predate the establishment of the Interstate system, according to the US Department of Transportation’s Federal Highway Administration.

  1. I-90: 3,101.77 mi (shown in red on the map)
  2. I-80: 2,899.54 mi
  3. I-40: 2,555.10 mi
  4. I-10: 2,460.34 mi
  5. I-70: 2,153.13 mi
  6. I-95: 1,925.74 mi
  7. I-75: 1,786.47 mi
  8. I-94: 1,585.20 mi
  9. I-35: 1,568.38 mi
  10. I-20: 1,539.38 mi