All About Interstate Highways

• Numbering Themes

  • Directions
    • Even Numbers travel east-west (I-4, I-8, I-10, I-12, etc.)
    • Odd Numbers travel north-south (I-5, I-15, I-17, I-19, etc.)
  • Significance
    • Routes ending in "0" are major east-west routes (I-10, I-20, I-40, I-70, I-80, I-90)
    • Routes ending in "5" are major north-south routes (I-5, I-15, I-25, I-35, I-55, I-65, I-75, I-85, I-95)
  • Orientation
    • Lower numbered routes are generally located in the south and west
    • Higher numbered routes are generally located in the north and east
    • As a result, the Interstate Highway system forms a grid with numerical designations increasing gradually from low to high - both from west to east and south to north.
    • Interstate Highways were intentionally numbered so that they would not conflict with the preexisting U.S. Numbered System; in fact, the intent was that no Interstate Highway and U.S. Highway would share the same number within the same state.
      • I-24 and U.S. 24 currently both exist in Illinois.
      • Current plans for extensions to I-49, I-69, and I-74 will result in those routes meeting and intersecting their U.S. Highway counterparts.
      • Interstate 50 and Interstate 60 were not assigned so as to avoid conflicts with U.S. 50 and U.S. 60 in the central part of the country.
    • Some Interstate Highways are Unsigned, including a variety of spur and loop routes that are often superfluous or confusing designations in addition to the primary route number.
    • Loop routes and through routes that generally connect to an Interstate highway at either end have an even first digit.:
      • Full 360-degree beltways include I-270 OH, I-275 KY/OH/IN, I-285 GA, I-410 TX, I-465 IN, I-495 MD/VA, I-610 TX, I-695 MD .
      • I-275 in KY, IN, and OH is the only three-digit Interstate currently to serve three states.
      • I-495 (Capital Beltway) serves Virginia and Maryland, but a portion of the route passes through the District of Columbia while crossing the Potomac River at the Woodrow Wilson Bridge.
      • I-271 and I-480 merge together briefly east of Cleveland, marking the only location where two three-digit Interstate routes merge together.
      • The following routes have an even first digit but do not end at another Interstate highway on one end: I-264 VA, I-295 NY, I-478 NY, I-495 NY, I-678 NY, I-635 TX, I-878 NY.
      • The following routes are examples of intercity routes, acting in a regional capacity: I-280 CA, I-476 PA, I-495 MA, I-680 CA, I-684 NY/CT
      • I-238 in CA acts as a spur of I-80, connecting I-880 and I-580 in the San Francisco Bay Area. At the time it was commissioned, no more I-x80 route numbers were available for use, so the designation of adjacent CA 238 was used to number I-238.
      • Some even-prefixed three-digit routes serve states that are not served by their parents, including I-275 OH/KY/IN and I-287 NJ/NY.
    • Spur routes and city routes that may or may not connect to an Interstate highway at one end have an odd first digit:
      • I-585 in SC does not connect to its parent (I-85) currently; it begins at Business Loop I-85 and ends in downtown Spartanburg.
      • The following routes are examples of intercity routes, acting in a regional capacity: I-380 PA, I-385 SC, I-505 CA.
      • Some odd-prefixed three-digit routes serve states that are not served by their parents, including I-129 NE/IA and I-535 MN/WI.
    • Some Interstates have state route extensions, including I-15 in San Diego (continues as CA 15), I-265 in New Albany (continues as IN 265), I-381 in Bristol (continues as VA 381), I-481 in Syracuse (continues as NY 481), I-690 in Syracuse (continues as NY 690), and I-794 in Milwaukee (continues as WI 794/Lake Parkway).
    • Some proposed future Interstate corridors have state route designations for now, including TN 840 near Nashville and CA 905 in San Diego.
    • Still other state routes have numbers that may make it look like a future Interstate corridor but are not, including VT 191 in Newport and IL 394 near Chicago Heights.