About the DOT

Iowa DOT's Tiffin WB rest area
DID YOU KNOW? The I-80 westbound
rest area in Tiffin features education-
themed design elements including books,
building blocks and an old school house.

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.
  • The exit number is determined by the milepost preceding the exit, in the direction of increasing mileposts.