Interstate highway numbering system

Interstate highways are numbered and mile-posted according to consistent national standards. Some of the rules are:

  • Even numbered interstates run east and west.

  • Odd numbered interstates run north and south.

  • Interstates with three-digit numbers are spur routes, bypasses or beltways.

  • Interstates with numbers ending in zero run from coast-to-coast.

  • Interstates with numbers ending in five run from border-to-border.

  • Interstate mileposts run from state line to state line, either from south to north or from west to east.

  • Interstate interchange numbers are assigned based on the nearest milepost. Where more than one interchange is located within a mile, letters are assigned to designate the individual interchanges.