How Highways are Numbered U.S Interstate Highways

U.S. Interstate Highway Numbering System

Major Highways

• Two-Digit Numbers:

  • Labeled with two digits that are divisible by five.

  • East-West Highways: End in zero (e.g., I-10, I-20, I-90).

    • No I-50 or I-60 to avoid confusion with previously existing U.S. highways.

  • North-South Highways: End in five (e.g., I-5).

    • I-5 is an exception; consider it as I-05 for consistency with the two-digit system.

Navigation and Numbering Logic

• Lower and Higher Numbers:

  • Numbers decrease as you move southwest.

  • Numbers increase as you travel northeast.

• Zero and Five Endings:

  • Zero at the end indicates horizontal movement (east-west).

  • Five at the end signifies vertical movement (north-south).

Minor Highways

• Three-Digit Numbers:

  • These are branches off major highways.

  • Last two digits match the corresponding major highway number.

  • Even First Digit: The minor branch returns to the major highway.

  • Odd First Digit: The minor branch does not connect back to the major highway.