Even numbered 3-digit interstates

[Mergingtraffic]

I have noticed on various road forums and even road-related Facebook pages where people would state: an even-prefixed 3-digit interstate has to be a loop or a beltway type deal.  Or it has to connect with the same interstate at both ends.  I just wonder where that comes from?

i bring it up as I saw this today.  There was talk about a northern portion of I-680 becoming I-880 in Iowa and a comment said "shouldn't it be an odd number b/c it doesn't reconnect with I-80?"
https://www.wowt.com/content/news/Northern-tier-of-I-680-to-be-renamed-I-880-563078321.html


From the FHWA it says this:

Connecting Interstate routes and full or partial circumferential beltways around or within urban areas carry a three-digit number. These routes are designated with the number of the main route and an even-numbered prefix. Supplemental radial and spur routes, connecting with the main route at one end, also carry a three-digit number, using the number of the main route with an odd-number prefix.

By looking at that statement I would say as long as both ends of a even-prefixed 3-digit interstate connect to another interstate, an even number is allowed.  So why do people and even fellow road-geeks believe it?

And here's an excuse to post a pic:

[sprjus4]

Ideally, an even number should be used when the roadway is linking two interstate highways, whether it be the same interstate highway at either end (I.E. a beltway, bypass, etc.), or another interstate highway (I.E. a connector).

An odd number should be used if it's leaving an interstate highway, and linking to an area / route that isn't another interstate highway (I.E a spur route to link one metro into the interstate system).

[hotdogPi]

It could go either way for 880. It's circumferential, not radial, with regard to Omaha (which means even), but it's also almost perfectly a straight line (which means odd).

[ilpt4u]

I tend to think it should be Odd 3DI when the Parent is only "touched"  once, but that view is at least partially colored because that is how they are designated in IL

In Illinois: 255, 270, 280, 290, 294, 474 all touch their Parent twice (255, 270, and 280 need the continuation of the route into neighboring MO and IA to accomplish this). 155, 172, 180, 190, 355 all touch their Parent once

By the idea that designations should be Even 3DIs when both endpoints are on the Interstate System, then 155 and 355 in IL should probably be 455 and 655. Supposedly 455 was considered for 355 at one point, even

Future 490/O'Hare Ring Tollway will endpoint at Parent I-90 and I-294, so it will be the first Even 3DI in IL that only touches its Parent once

Going from your Pic...PA is messed up with 376 and 476...376 is more like a Loop for I-76, touching the Parent twice, and I-476 is more like a Spur North/South from I-76, touching the Parent only once

[Eth]

but it's also almost perfectly a straight line (which means odd).

Counterpoint: I-475 (GA) is nearly a straight line, but wouldn't make any sense at all with an odd first digit.

[Hot Rod Hootenanny]

All of Ohio's 3di are even numbers. Only I-270, I-275, I-470, and I-480 are either full beltways, or (at least) meet its parent on both ends.

[ilpt4u]

All of Ohio's 3di are even numbers. Only I-270, I-275, I-470, and I-480 are either full beltways, or (at least) meet its parent on both ends.

Toledo's 475 endpoints on I-75, both ends, too

[US71]

I-440 in Arkansas doesn't connect to 40 on both ends, though it is a beltway.

I-430 runs from I-40 to I-30, I-630 runs from I-430 to I-30.

[Max Rockatansky]

There is a tendency for even digit numbered routes to connect back to their parent but out here in California that trend is non-existent.   A good example is the I-80 family of highways in California:

-  I-280 doesn't even connect to it's parent but does by proxy to I-880 and I-680
-  I-480 connected to it's parent but didn't loop back to it.
-  Likewise I-680 and I-880 touch their parent route I-80 but definitely aren't loops. 

Interestingly though I-405 and I-805 do loop to/from I-5 whereas none of the odd number child routes do.  I-215 does loop to I-15 but that route was intended to be I-15E.

[kurumi]

That guideline used to be honored more than it is now.

Example: long ago, the announced plan was to redesignate I-540 in NC as I-640 once it was completed; but that idea is now dead. The benefit of following the system is probably overshadowed by the drawback of renumbering a freeway.

I-680 to I-880 in Iowa is a bit of a different case in that the renumbering purpose is to decrease driver confusion. I think, in an alternate universe, if that segment had been originally numbered I-780 there would be no pressure (other than some roadgeeks) to "correct" it to 880.

[US 89]

One exception that comes to mind is I-526, which is a partial circumferential beltway around Charleston and as such should probably have received an even first digit. It’s not like SC was going to run out of even x26 numbers, either.

[Eth]

That guideline used to be honored more than it is now.

Example: long ago, the announced plan was to redesignate I-540 in NC as I-640 once it was completed; but that idea is now dead. The benefit of following the system is probably overshadowed by the drawback of renumbering a freeway.

And also, AFAIK there are no plans to renumber I-520 (GA/SC) even though it now reconnects with its parent, presumably for the same reason.

[Brandon]

I have noticed on various road forums and even road-related Facebook pages where people would state: an even-prefixed 3-digit interstate has to be a loop or a beltway type deal.  Or it has to connect with the same interstate at both ends.  I just wonder where that comes from?

It varies by state.  Some states only use and even prefix on those beltways and similar route, leaving anything that is a spur to have an odd prefix.  Illinois is one of those states.

I-55
I-155: Connects I-55 to I-74, but never meets I-55 again.
I-255: Connects to I-55 at two ends (one in Missouri), and continues to meet I-270.
I-355: Connects I-55 to I-80 (south) and I-88 and I-290 (north), but only meets I-55 once.

I-70
I-270: Connects to I-70 twice, once in Missouri.

I-72
I-172: Connects to I-72 once, and spurs to a divided highway.

I-74
I-474: Connects to I-74 at both ends.

I-80
I-180: Spurs off I-80 to two lane roads.
I-280: Connects to I-80 at both ends (one in Iowa) and has a concurrency with I-74 just to meet I-80.

I-90
I-190: Spurs off I-90 into O'Hare.
I-290: Connects to I-90 at both ends.

I-94
I-294: Connects to I-94 at both ends and has a concurrency with I-80 just to meet I-94.  Before I-90 and I-94 were switched, I-294 went into Indiana just to meet I-94.

[paulthemapguy]

I think that using even numbers in cases like I-880, with only one connection to its parent, makes it too difficult to create odd-starting 3dis.  Here we go again with a state's even 3di's getting used up, when we haven't used half of the odd Iowa x80's.  What if the Des Moines outer beltway becomes Interstate standard?  I guess that will have to be an x35?  There haven't been any even x29s used in Iowa...

[PHLBOS]

From the FHWA it says this:

Connecting Interstate routes and full or partial circumferential beltways around or within urban areas carry a three-digit number. These routes are designated with the number of the main route and an even-numbered prefix. Supplemental radial and spur routes, connecting with the main route at one end, also carry a three-digit number, using the number of the main route with an odd-number prefix.

By looking at that statement I would say as long as both ends of a even-prefixed 3-digit interstate connect to another interstate, an even number is allowed.

To this day, many Rand McNally maps & atlases describe even 3-digit Interstates as:
First-digit even: Routes go through or around a city

That said & based on the above-descriptions; I would be inclined to believe that even-prefixed 3dis that aren't complete/full beltways don't necessarily have to end at its parent 2di at both ends.

That guideline used to be honored more than it is now.

The likely reasoning for such is the result of some states running out of available even prefixes; see the recent extended I-376 scenario in PA.

In retrospect, it might have been better to have swapped the odd/even-prefixed criteria for 3dis.  Such was the reasoning for my starting this Fictional thread a while back.

[Mark68]

In Colorado, neither even-prefixed interstate loops or connects to its parent more than once.

I-225 connects to I-25 on the south and I-70 on the north, whereas I-270 connects to I-25 on the west and I-70 on the east.

The 470s (C-470 & E-470) serve as more of a loop (as they were originally intended to be designated as I-470) than the ones that actually have a blue and white shield.

[Brandon]

I think that using even numbers in cases like I-880, with only one connection to its parent, makes it too difficult to create odd-starting 3dis.  Here we go again with a state's even 3di's getting used up, when we haven't used half of the odd Iowa x80's.  What if the Des Moines outer beltway becomes Interstate standard?  I guess that will have to be an x35?  There haven't been any even x29s used in Iowa...

Personally, I thought it should be an I-x29, maybe I-829 to symbolize the connection between I-80 and I-29.

[GaryV]

Michigan has a few odd 1st digit 3di's - 194, 196 and 375.  All originally were spurs.  I-196 was swapped with the westernmost portion of I-96 when what is now I-196 was not yet completed so a Detroit-Muskegon single route made more sense than an incomplete Detroit-Grand Rapids-Benton Harbor route.

Most even 1st digit routes in Michigan connect to their parent at both ends, with the exception of I-275 (although it was originally planned to) and I-696.  Also the unmarked I-296, but that does connect I-96 and I-196 so it sort of meets the non-existent "connect at both ends" rule.  But the exceptions connect to other Interstates at each end.

[DTComposer]

There is a tendency for even digit numbered routes to connect back to their parent but out here in California that trend is non-existent.   A good example is the I-80 family of highways in California:

-  I-280 doesn't even connect to it's parent but does by proxy to I-880 and I-680
-  I-480 connected to it's parent but didn't loop back to it.
-  Likewise I-680 and I-880 touch their parent route I-80 but definitely aren't loops. 

Interestingly though I-405 and I-805 do loop to/from I-5 whereas none of the odd number child routes do.  I-215 does loop to I-15 but that route was intended to be I-15E.

I-605 is the strangest of the bunch, intersecting I-5 basically mid-way through its route. It connects to Interstates on each end, but IMO acts more like two spurs (from I-5 north, I-605 north spurs to the San Gabriel Valley; from I-5 south, I-605 south spurs to coastal Orange County).

[Mergingtraffic]

From the FHWA it says this:

Connecting Interstate routes and full or partial circumferential beltways around or within urban areas carry a three-digit number. These routes are designated with the number of the main route and an even-numbered prefix. Supplemental radial and spur routes, connecting with the main route at one end, also carry a three-digit number, using the number of the main route with an odd-number prefix.

By looking at that statement I would say as long as both ends of a even-prefixed 3-digit interstate connect to another interstate, an even number is allowed.

To this day, many Rand McNally maps & atlases describe even 3-digit Interstates as:
First-digit even: Routes go through or around a city

That said & based on the above-descriptions; I would be inclined to believe that even-prefixed 3dis that aren't complete/full beltways don't necessarily have to end at its parent 2di at both ends.

That's what I'm saying, even people in this thread think the "new" I-880 should be an odd 3di.  I don't think it should as it connects two different interstates.  I don't know where the Rand McNally thinking came from b/c the FHWA doesn't say that.

[kphoger]

I don't know where the Rand McNally thinking came from b/c the FHWA doesn't say that.

Seriously.  I mean, if that were true, then I-238 would...

[PHLBOS]

I don't know where the Rand McNally thinking came from b/c the FHWA doesn't say that.

Not sure of its origin; but such has been on many of its maps & atlases for decades. 

Aside from Rand McNally not mentioning connections to Interstates at both ends (if not a full beltway), where does its definition differ from the FHWA's definition?  There are examples of even 3dis going through cities whereas its parent route bypasses it.  I-295 going through Portland, ME vs. I-95/ME Turnpike bypassing Portland being one example.  Note: Such existed decades before the I-95/295 swap further north of Portland.

For comparison purposes, both definitions (FHWA & Rand McNally) are reposted below with bold emphasis added:

Connecting Interstate routes and full or partial circumferential beltways around or within urban areas carry a three-digit number. These routes are designated with the number of the main route and an even-numbered prefix. Supplemental radial and spur routes, connecting with the main route at one end, also carry a three-digit number, using the number of the main route with an odd-number prefix.

First-digit even: Routes go through or around a city

[kphoger]

Aside from Rand McNally not mentioning connections to Interstates at both ends 

That's precisely it.  RMN says an even-numbered 3di has to go through or around a city.  That is one possible reading of the FHWA guidance:

(Connecting Interstate routes and full or partial circumferential beltways) (around or within urban areas)

However, I don't think that's the correct interpretation of the guidance.  I think instead, the guidance should be parsed thus:

(Connecting Interstate routes) (and full or partial circumferential beltways around or within urban areas)

That leaves open the possibility of a rural 3di.  See I-135 in Kansas for a good example of that.

[hotdogPi]

Radial/circumferential is my preference, but there are still a few problems, like I-355 in Illinois, which I feel is a correct odd number contrary to my preference.

[Konza]

I-880 is technically a spur that shortens the distance from Des Moines to Sioux City.

Even though it connects to another interstate, it isn't a belt route or a route through a city.

The first digit of the route number should be odd, whether the last two digits of the route number are 29 or 80.