I thought I basically had the interstate numbering system figured out, but I wasn't aware that you could have 2 highways with the same number in different parts of the country. An example, I-84 starts at a jct with I-5 in Portland Oregon & runs easterly to a jct with I-80 east of Salt Lake city & apparently ending. Then there is I-84 that appears to start at a jct with I-81 in Scranton, PA & travels east to a jct wih I-90 in MA & ends again. It just made me go hmmm, as there are 2 ends & no apparent middle section of this Interstate highway, unlike I-80 which starts in San Francisco & runs continuously eastward (occasionally merging with other interstate hgwys)to a jct with I-95 near Hackensack, NJ., or I-90 which starts in Boston & runs westward, again continuously, all the way to downtown Seattle. This is probably of no great concern in the scheme of life, but just made me wonder why. :@ PS: I know about by-passes being numbered the same in different areas.

I-80 hasn't always been complete. I remember when it had gaps in it too.

There is a ranking of numbers. The 10s and 5s were used for cross country runs, at least to the extent it made sense. Other even numbers were used for partial runs WE runs, and odds for NS. Where possible numbers are used in consecutive order (small to the south and west). As your example shows, it's not against the rules to have discontinuous segments, though those mostly are the result of name changes.In the East I see 84, 86 and 88 between 80 and 90. I don't seen an 82. In the west there's 86, 84 and 82. Given that there are only 4 choices between 80 and 90, so sort of reuse was inevitable. On further thought, 20, 30, 50, 60, 70 don't make it to California.http://en.wikipedia.org/wiki/Interstate_84_(Oregon%E2%80%93Utah)I84 was originally I80N, branch of I80 serving the PNW. The name change was in 1980, following a change in policy regarding the letter suffix like 'N'.http://en.wikipedia.org/wiki/Interstate_82The change to I84 put the numbering of I82 out of place (north of a larger number). I82 was originally planned to follow US12 to Tacoma, and there was even talk going south in Oregon along US97.http://en.wikipedia.org/wiki/Interstate_86_(west)I86 in Idaho was originally I15W. Again a name change to eliminate the letter suffix, even it meant 'duplicating' numbers elsewhere. I15E in southern California became I215.Curious fact - there is now a I2 - a short segment in the lower Rio Grande valley.

The numbering system is explained HERE. Scroll down about a third of the page to Numbering System.Clear as mud. :)

Even numbers..........east/westOdd numbers...........north/south

I-84's "split personality" is unique in the interstate system and has nothing to do with the east/west-north/south numbering system.The western segment began life as "I-80N", but somewhere along the line some folks with nothing better to do than mess with our heads decided there was something wrong with that designation/enumeration. After much deliberation apparently involving a truckload of Jim Beam, it was decided that I-80N would be renamed "I-84" despite the fact that a road with that name/number already existed back East or, as known here in the Pacific Northwest, "nowheresville". Note:Some Historians believe that the Commission in Charge of Doing Stupid Useless Things was simply unaware of that fact. Others say they did know, but thought it a Great Joke to go ahead and use it again anyway. See this link.

Pondering Interstate Highways | Good Sam Community

24 Replies

Old-Biscuit
Explorer III
Mar 03, 2014
Even numbers..........east/west
Odd numbers...........north/south
OH48Lt
Explorer
Mar 03, 2014
The numbering system is explained HERE. Scroll down about a third of the page to Numbering System.

Clear as mud. :)
paulj
Explorer II
Mar 03, 2014
There is a ranking of numbers. The 10s and 5s were used for cross country runs, at least to the extent it made sense. Other even numbers were used for partial runs WE runs, and odds for NS. Where possible numbers are used in consecutive order (small to the south and west). As your example shows, it's not against the rules to have discontinuous segments, though those mostly are the result of name changes.

In the East I see 84, 86 and 88 between 80 and 90. I don't seen an 82. In the west there's 86, 84 and 82. Given that there are only 4 choices between 80 and 90, so sort of reuse was inevitable.

On further thought, 20, 30, 50, 60, 70 don't make it to California.

http://en.wikipedia.org/wiki/Interstate_84_(Oregon%E2%80%93Utah)
I84 was originally I80N, branch of I80 serving the PNW. The name change was in 1980, following a change in policy regarding the letter suffix like 'N'.

http://en.wikipedia.org/wiki/Interstate_82
The change to I84 put the numbering of I82 out of place (north of a larger number). I82 was originally planned to follow US12 to Tacoma, and there was even talk going south in Oregon along US97.

http://en.wikipedia.org/wiki/Interstate_86_(west)
I86 in Idaho was originally I15W. Again a name change to eliminate the letter suffix, even it meant 'duplicating' numbers elsewhere. I15E in southern California became I215.

Curious fact - there is now a I2 - a short segment in the lower Rio Grande valley.
DavisK
Explorer
Mar 03, 2014
I-80 hasn't always been complete. I remember when it had gaps in it too.

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Pondering Interstate Highways

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