3 Digit Interstate Spur Route #s

[bluecountry]

How do they designate what #s these spur routes get?
I know essentially if it loops or bypasses it gets even digits, if it spurs, it gets odd.

But:
-How do they determine if it gets a low digit (1/2/3) vs high digit (7/8)
-If a spur interstate say meets 1 interstate, then terminates at a different interstate, is it even or odd digits?
    -What if it doesn't terminate at another interstate, but simply meets another interstate, but terminates elsewhere?

[hotdogPi]

-How do they determine if it gets a low digit (1/2/3) vs high digit (7/8)

There's no rule for it, but in general, the ones created later have higher digits. There are many exceptions, though.

[bluecountry]

-How do they determine if it gets a low digit (1/2/3) vs high digit (7/8)

There's no rule for it, but in general, the ones created later have higher digits. There are many exceptions, though.

Gotcha, so if you have 3 beltways, you could them be 285, 885, 485 or 485, 685, 285 from inner to outer?

[TheHighwayMan3561]

A lot of it also likely has to do with previously existing numbers in a state. In Minnesota, 194 came into use as a remembering of MN 94 and MN 294 was already in use, so 494 and 694 were the first available numbers for even numbered Twin Cities 3dis (I-394 was authorized later). Similarly, as MN 35 became MN 135, that left I-335 as the first number for the cancelled I-35W downtown bypass route.

[GaryV]

Often the first number of the 3DI is in order from west to east / south to north in a state.  This only works when the 3DI's were established at the same time as the 2DI they branch off.

[nexus73]

Oregon's first was I-105 in Eugene.  I-305 was supposed to be for Salem with that proposed route being replaced by the Salem Parkway.  I-505 was supposed to be in PDX and come off of I-405.  A small piece of freeway was built but it kept the US 30 designation. 

There never has been any I-705/905, I-x84 or I-x82 proposals made for Oregon AFAIK.

Rick

[formulanone]

Sarcastically speaking: a 1xx route is a duplicate of a state route, 3xx actually gets used, 5xx gets cancelled, 7xx is either pointless or in development hell, and 9xx will never happen in our lifetimes.

(Except for Georgia and California, but the latter will never build another interstate route.)

[CoreySamson]

Often the first number of the 3DI is in order from west to east / south to north in a state.  This only works when the 3DI's were established at the same time as the 2DI they branch off.

I might also add that even 3DIs and odd 3DIs are (or are supposed to be) independently ordered from west to east. Instead of seeing a 1xx, then a 2xx, then a 3xx; you might see a 2xx, then a 4xx, then a 1xx.
Great example of this is I-10 in Louisiana. Lake Charles has a 210, Baton Rouge was supposed to have a 410, and New Orleans has a 610; however, Baton Rouge has a 110 to the east of 210.

Like others have said, this has many exceptions, but I would like it if this became the universal standard for 3DIs.

[CtrlAltDel]

I might also add that even 3DIs and odd 3DIs are (or are supposed to be) independently ordered from west to east. Instead of seeing a 1xx, then a 2xx, then a 3xx; you might see a 2xx, then a 4xx, then a 1xx.
Great example of this is I-10 in Louisiana. Lake Charles has a 210, Baton Rouge was supposed to have a 410, and New Orleans has a 610; however, Baton Rouge has a 110 to the east of 210.

Like others have said, this has many exceptions, but I would like it if this became the universal standard for 3DIs.

I would not. As a general rule, the lower numbers are assigned before higher numbers, which leads to a relative glut of lower number 3di's that is unnecessary, and potentially confusing, as in the eight I-295's (most of which are found within a few hundred miles in the Northeast) compared to the 4 instances of I-695 and I-895 combined. Spreading things out more evenly seems wise, to my mind.

[oscar]

9xx will never happen in our lifetimes.

(Except for Georgia and California, but the latter will never build another interstate route.)

I-990 in New York, too.

As for California, it has CA 905 already built as freeway from I-5 to the Mexican border. But Caltrans doesn't really give a crap about pursuing new Interstate designations.

[KeithE4Phx]

I live in Arizona.  What is it you mean by "three digit Interstate?" 

[Revive 755]

Sarcastically speaking: a 1xx route is a duplicate of a state route, 3xx actually gets used, 5xx gets cancelled, 7xx is either pointless or in development hell, and 9xx will never happen in our lifetimes.

In a similar vein for even three digits: 4xx gets renumbered or absorbed by another route, 6xx gets cancelled. 

[CoreySamson]

I might also add that even 3DIs and odd 3DIs are (or are supposed to be) independently ordered from west to east. Instead of seeing a 1xx, then a 2xx, then a 3xx; you might see a 2xx, then a 4xx, then a 1xx.
Great example of this is I-10 in Louisiana. Lake Charles has a 210, Baton Rouge was supposed to have a 410, and New Orleans has a 610; however, Baton Rouge has a 110 to the east of 210.

Like others have said, this has many exceptions, but I would like it if this became the universal standard for 3DIs.

I would not. As a general rule, the lower numbers are assigned before higher numbers, which leads to a relative glut of lower number 3di's that is unnecessary, and potentially confusing, as in the eight I-295's (most of which are found within a few hundred miles in the Northeast) compared to the 4 instances of I-695 and I-895 combined. Spreading things out more evenly seems wise, to my mind.

Good point. However, I can see its use in larger states that have fewer 3dis. Perhaps 4dis would be an idea in those more densely populated states to avoid confusion? (i.e, 295 in DC becomes 1295 and 295 in New Jersey becomes 2295. Just spitballing here.)

[Beltway]

Digits usually run from low to high in a state.

VA I-195, I-295, I-395, I-495.  Unused/canceled I-595.  Unused I-895.

VA I-264, I-464, I-564, I-664 -- the even numbers run in sequence, but the odd numbers (spurs) start with 564.

VA I-381, I-581.  I-181 was in Tennessee but was changed.

VA I-266 -- Unused/canceled

[TravelingBethelite]

Digits usually run from low to high in a state.

VA I-195, I-295, I-395, I-495.  Unused/canceled I-595.  Unused I-895.

VA I-264, I-464, I-564, I-664 -- the even numbers run in sequence, but the odd numbers (spurs) start with 564.

VA I-381, I-581.  I-181 was in Tennessee but was changed.

VA I-266 -- Unused/canceled

Don't forget I-366

[oscar]

Good point. However, I can see its use in larger states that have fewer 3dis. Perhaps 4dis would be an idea in those more densely populated states to avoid confusion? (i.e, 295 in DC becomes 1295 and 295 in New Jersey becomes 2295. Just spitballing here.)

Take a look at one of Hawaii's Interstate H-201 markers:



Even using an unpleasantly compressed font, and with one skinny number, four numbers would be a rather tight fit on a wide Interstate route marker.

[Roadgeekteen]

All the rules listed in this thread (except the even/odd rule, but even that has exceptions) have multiple exceptions, such as I-985 in Georgia.

[ilpt4u]

IL keeps it simple - Pretty much Low to High, Evens and Odds

I-155, I-255, I-355

I-180, I-280

I-190, I-290, IL 390, Future I-490

Former IL 194, I-294, IL 394, Cancelled I-494

I-172

I-270

I-474 in Peoria is the slight IL oddball, with no IL I-274. https://www.kurumi.com/roads/3di/ix74.html tells us that 274 could have been used for part of I-80 in the Quad Cities area, between the 2 I-74 junctions. I-80 would have followed I-74 thru Downtown Moline and Bettendorf

[Beltway]

Digits usually run from low to high in a state.
VA I-195, I-295, I-395, I-495.  Unused/canceled I-595.  Unused I-895.
VA I-264, I-464, I-564, I-664 -- the even numbers run in sequence, but the odd numbers (spurs) start with 564.
VA I-381, I-581.  I-181 was in Tennessee but was changed.
VA I-266 -- Unused/canceled

Don't forget I-366

That could be a candidate for VA-28 after it is all a full freeway between I-66 and VA-7.

That is a well-developed corridor and serves alongside of Dulles Airport.

Maryland
I-270, I-370.   I-170 dedesignated.
I-195, I-295, I-395, I-495, I-695, I-795, I-895.   I-595 unsigned.

[ErmineNotyours]

This gets to an issue I've brought up before.  I think Tacoma's I-705 is so named because it begins and ends at Washington SR-7, but someone keeps saying it's a coincidence.  The only reason it could be a coincidence is if Washington is reserving numbers for three more spur Interstates south of Tacoma.

[sprjus4]

That could be a candidate for VA-28 after it is all a full freeway between I-66 and VA-7.

That is a well-developed corridor and serves alongside of Dulles Airport.

Given VDOT's track record, they'll find a way to say it doesn't meet interstate standards.

[hotdogPi]

I would prefer the first digit to be by importance, where e.g. I-1XX is a medium-length connector. I-135 in KS and I-155 in Illinois are good examples of this already existing. Unfortunately, there are so many short I-1XX routes this it doesn't form a pattern.

[cwf1701]

Michigan:
75: I-275, I-375, I-475, I-675
94: I-194
96: I-196, I-296, I-496, I-696

[Hwy 61 Revisited]

PA:
I-80: 180, 380.
I-76: 376, 576, 176, 476, 276, 676.
I-95: 295.
I-83: 283.
I-81: 581.
I-79: 279, 579.
I-99: No auxiliary routes.

[Roadgeekteen]

We ever going to get 4 digit interstates?