The methodology behind US route numbers
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US highway numbering conventions stipulated that 2-digit ("main") US routes running generally north/south were to be assigned odd numbers, beginning on the east coast and increasing to the west:
Similarly, even numbers went to east/west routes, beginning in the north and increasing to the south. However, some exceptions existed even when the system was first proposed. As the Board responsible for the numbering system noted, "An unbroken numerical sequence was not possible unless lines of prevailing flow of traffic were to be entirely neglected. Such lines cross each other and demand that numerical order be sacrificed in a few cases". And as these maps illustrate, subsequent additions, extensions, and truncations were not always in harmony with the numbering guidelines:
Three-digit numbers were (usually) used for shorter routes that functioned as spurs or branches off the main routes. The last two digits identified the "parent" route to which the "child" route was attached. For example, this next map shows US 30 and its branch routes, beginning with US 130 in the east, and increasing to the west with US 230, 330, and so on, up to US 830 (although it should be noted that not all branch routes were numbered in geographical sequence). US 30 is shown in purple; current branch routes are blue; and historic routes are red:
Unlike 2-digit US routes, there was never a guideline stating that 3-digit routes should run in a particular direction -- that is, no stipulation that north/south branch routes should be assigned odd numbers (so as to be considered branches of north/south main routes), and that even numbers should be given to east/west branch routes. Perhaps the reason was that in some cases there was no appropriate number available. For example, there are some east/west branch routes that connect only with north/south main routes. So they cannot be considered branches of east/west routes, and an even number would not make sense.
Despite the fact that there was no "official" methodology for numbering 3-digit branch routes, one might still wonder: wherever possible, has the parity-direction concept also been followed among 3dus routes? The following maps indicate that, for the most part, it has. These maps illustrate how most north/south 3dus routes have been assigned odd numbers, and in most cases even numbers have been given to east/west 3dus routes.
Despite the fact that there was no "official" methodology for numbering 3-digit branch routes, one might still wonder: wherever possible, has the parity-direction concept also been followed among 3dus routes? The following maps indicate that, for the most part, it has. These maps illustrate how most north/south 3dus routes have been assigned odd numbers, and in most cases even numbers have been given to east/west 3dus routes.
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Legend
Purple: current 2dus routes Blue: current 3dus (and some alternate) routes Red: historic and/or decommissioned routes This map shows all odd-numbered 2dus and 3dus: |
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This map shows all even-numbered 2dus and 3dus:
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Research and/or image credits: Dale Sanderson
Page originally created 2003;
last updated Feb. 28, 2025.
last updated Feb. 28, 2025.
