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The American Association of State Highway and Transportation Officers. They are the ones who set the standards for numbering US Routes and Interstates. Below are the AASHTO Standards.

Numbering Convention for United States Numbered Highways

The U.S. Numbered Highway System is not a true Federal program of roadways. It is a series of interconnected state highways with a common numbering plan for the purpose of aiding navigation. They are built and maintained by the individual states, and funded in the same manner as any other state highway. In most states, they are simply a state highway with a different shield. Numbering and signing of US routes is approved by the American Association of State Highway and Transportation Officers (AASHTO). Participation the the US route system is voluntary. States are not bound by Federal law to comply with US route designations.

  • US routes are all in the contiguous 48 states and have 1, 2, or 3 digit numbers.
  • Even numbers run east to west; increasing to the south
    • Principal routes are one or two digits (abbreviated as 2dus) and end in 0: US 10 to US 90
    • Existing range is US 2 to US 98
  • Odd numbers run north to south; increasing to the west
    • Principal routes are one or two digits (with a single three digit exception) and end in 1: US 1 to US 101
    • Existing range is US 1 to US 101
  • Increments of low-order digits fill in the grid
    • e.g. US 20, US 22, US 24, ... US 30 - Even Numbered
    • e.g. US 21, US 23, US 25, ... US 31 - Odd Numbered
  • In addition, an increment of 3rd high order digits (3dus) denote branch highways or spur routes.
    • e.g. (US 66), US 166, US 266, (US 366), (US 466), (US 566), US 666
  • US Divided-number Routes are of the form nd where d is {N|S|E|W}
    • e.g. US 70N, US 70S and (US 99E), (US 99W).

  • In 1970, AASHTO revised its guidelines for US highways.
  • No new divided route numbers; Existing ones should be eliminated.
  • No new numbers for routes wholly in a single state.
  • Existing single state routes < 300 miles long should be eliminated.

Additionally, auxilliary signs further define the US Highways ... Sometimes these banners are signed as suffixes, like US 89A which means Alternate US 89 Cardinal directions, such as -WEST- are placed above the shield. These banners indicate additions to the US Highway system

ALTERNATE ALT (OPTIONAL) (OPT) : a longer or less well constructed loop along the main route which would have been suitable as the mainline if a better route was not available. OPT is an old name for this type.
BUSINESS BUS (CITY) : a loop leading through the heart of a city's commercial area, generally the original route though town. City is the older name for the type.
BYPASS BYP RELIEF (BELT LINE) : a loop leading around the heart of a city's commercial / most congested area, generally more recently built. BELT LINE is an older little-used name for the type.
DETOUR : an interim route used while the main route is under construction. Usually used for mainline traffic. Uses an orange banner.
HISTORIC OLD FORMER : previous alignments, not recognized by AASHTO.
SCENIC : a little used subset of ALT.
SPUR CONNECTOR CONN : a branch route connecting to a specific location along the road. Connector routes, a subset of the type, have been appearing recently
TEMPORARY TEMP : an interim route used while the main route is under initial construction, or when the proposed routing is not yet finalized. Used for mainline traffic.
TOLL : a route loop you have to pay to drive on. Rarely used with US routes, requires a free US signed alternative.
TRUCK : a subset of Bypass meant for trucks and other large vehicles.

Extracted From http://www.us-highways.com/

Numbering Conventions for the United States Interstate System

Unlike US Highways, states are required to follow the Interstate numbering system since unlike the highways, there is a law for it. Also, there exists non-chargeable and chargeable interstates, chargeable interstates get 90% federal funding while non-chargeable only get 10%.

  • Even numbers run east-west and increase to the north.
  • Odd numbers run north-south and increase to the east.
  • Three-digit Interstates are spurs off major interstates an can be repeated.
    • A hundreds digit of 1 is a local spur into a city.
    • A hundreds digit of 2 is an express through a city.
    • A hundreds digit of 3 is a bypass between highways.
    • A hundreds digit of 4 is an express around a city.
    • A hundreds digit of 5 is a local spur into a city.
    • A hundreds digit of 6, 7, or 8 is a wide bypass around a metropolitan area
  • Letter Suffix Routes are not allowed (except under certain circumstances) since the rewriting of the rules in 1980.
  • Letter Prefixes Routes are used for Hawaii (H-1, H-2, and H-3) and Alaska (A-1, A-2, A-3, and A-4). Although they're not "interstates" they're their for getting federal funds.

Basically AASHTO just switched the highway numbering system around.

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