Who decides who gets to have kids?

[hbelkins]

I've often wondered how it's decided which interstate gets to be the "parent' when there are multiple routes to choose from.

Example: Birmingham, Ala. You have I-20, I-59, and I-65. Yet I-59 is the parent of the three-digit bypass route.

Likewise, Atlanta, where I-20, I-75, and I-85 are present, but 75 and 85 have children but 20 doesn't.

So it's obviously not the lowest-numbered interstate.

[JayhawkCO]

Most of the time, it's the women who decide.

[SeriesE]

Maybe the one that the aux route has the most parallel mileage with?

In the Atlanta example, the loop is more oblong towards following I-85

In the Birmingham example, it parallels I-59 more and extends east beyond I-20 to end at I-59.

[wanderer2575]

Maybe the one that the aux route has the most parallel mileage with?

I-280 in Ohio shoots down that theory.

[Dirt Roads]

I've often wondered how it's decided which interstate gets to be the "parent' when there are multiple routes to choose from.

Example: Birmingham, Ala. You have I-20, I-59, and I-65. Yet I-59 is the parent of the three-digit bypass route.

Likewise, Atlanta, where I-20, I-75, and I-85 are present, but 75 and 85 have children but 20 doesn't.

So it's obviously not the lowest-numbered interstate.

I'm not sure about 3DIs in the original system, but afterwards most states picked a number based on where the new freeway was originating.  For instance, Ohio originally planned I-471 starting from close to where it connects to I-71 in Cincinnati and it was supposed to cross its way into Kentucky and work back west to connect with I-75/I-71 (where you could get to pick a different parent). 

Pittsburgh presented a different set of critters, with I-70 being originally routed downtown on the Parkway East and the Parkway East; later the Parkway East became I-76 and the Parkway West became I-79.  When I-79 swapped with I-279 to bypass Pittsburgh, the Parkway West became an extension of I-76.  After that, it was certainly appropriate to number the route as I-376.  After all of the later extensions, it would look weird being numbered as an I-X79 when it connects to the Pennsylvania Turnpike twice.

IIRC, the first section of I-459 was connected to I-59.  Once completed, it too connects to its parent twice.  But I can't find any of those details and I'm sure I don't have the old maps anymore.  I used to remember these kinds of details and kept track of the weird exceptions (at least east of the Mississippi).

[freebrickproductions]

Likewise, Atlanta, where I-20, I-75, and I-85 are present, but 75 and 85 have children but 20 doesn't.

IIRC, I-20 was planned to have an I-420 off of it in Atlanta.

In Birmingham, I-22 is also getting an I-222 and I-422. That being said, I've always found it interesting that the spur in Tuscaloosa is I-359 and not I-320. My guess is that they wanted an odd number due to it being a north/south spur, or they wanted it to match the other two I-59 daughters.

[NE2]

The states submitted numbering to AASHO, so they got to choose. AASHO only got directly involved when routes crossed state lines, didn't follow numbering, or were otherwise janky (Iowa wanting an x35 and x80 overlapping through Des Moines).

[DandyDan]

The states submitted numbering to AASHO, so they got to choose. AASHO only got directly involved when routes crossed state lines, didn't follow numbering, or were otherwise janky (Iowa wanting an x35 and x80 overlapping through Des Moines).

I can see why the Interstate through Des Moines proper is I-235. I-80 originally had three x80's that crossed state lines, so it's just easier to make that one an x35.

[formulanone]

Most of the time, it's the women who decide.

[ mandatory Bud Shuster joke ]

I have a pet theory that a metro area wants a unique identifier, if they can.

But...maybe others just don't care: "gimme another 110!" Most business leaders and/or whatever pencil pusher crafts the idea isn't necessarily involved in a lot of deep research about the choice.

In the case of Birmingham, 459 was available; Jackson, Mississippi had 220 and Atlanta had 420 proposed.

[Beltway]

Most of the time, it's the women who decide.

Correct -- she is the gatekeeper.

[Max Rockatansky]

Child routes is what happens when Interstates don't wear rubbers.

[Big John]

Child routes is what happens when Interstates don't wear rubbers.

That would be tire-some.

[vdeane]

I imagine it's picked based on what seems more logical.  I-459 ends at I-59 on both sides, while it crosses over I-20 with only one end ending at I-20.  I'm not sure what the story with I-285, but it makes me wonder if the section where they parallel each other used to be an overlap or gap.  I'm not sure what the story with I-359 is, but between the through movements on the AL side and the exit numbers on the MS side, I-59 seems to be the primary route on the I-20/59 overlap.

[ilpt4u]

There is the St Louis Solution: Part of the Beltway is I-270. The rest is I-255. Historically there was also a segment designated I-244 but those days are long past and that segment is part of modern I-270

I-55 and I-70 are the x5 and x0 2dis and therefore get the kids

In some parallel universe somewhere I-64 or even I-44 might have been christened I-50 or 60, but not in this one.

[Henry]

In Chicago, I-294 makes up part of the Tri-State Tollway, and I-355 is the North-South Tollway. Before I-290 came along, it was the original route of I-90, but after I-90 was realigned, it ran on both the Dan Ryan south of downtown and the Kennedy north of it, with I-290 being the dividing line. I-490 is on the way too, so every Chicagoland 2di (except for I-57 and the latecomer I-88) is entitled to have kids.

[Revive 755]

Maybe the one that the aux route has the most parallel mileage with?

I-280 in Ohio shoots down that theory.

I-280 could be an exception - maybe Ohio wanted to keep 875 in reserve while avoiding an overlap of I-75 and I-475 - the latter of which could have been extended to take over I-280?

There's also I-220 in Mississippi which looks more parallel and more of a bypass for I-55.  Perhaps Mississippi didn't want to duplicate I-255 (which had been present in the Memphis area), but I-455 would have been available.

There also could have been I-210 in Mobile, Alabama (had what is I-165 been built all the way to I-10) which would have looked more parallel with I-65 than I-10.

Though the 'most paralleling mileage' still looks like a fairly good explanation in a number of cases.

[LilianaUwU]

I think it's based on what the DOT feels like choosing that day.

[freebrickproductions]

I'm not sure what the story with I-359 is, but between the through movements on the AL side and the exit numbers on the MS side, I-59 seems to be the primary route on the I-20/59 overlap.

I also suspect it's because I-359 runs north/south, and the state wanted a number that matched the direction it ran. This is the same state that originally signed I-565 as north/south originally, after all.

[achilles765]

Which is why some of them here in Texas are curious to me. We don't have very many 3dis (though I think we could easily get at least 5 or 6 more).
IH 110 in El Paso makes sense—it's the only 2di there.

But why IH 410 and 610 and not IH 235, 435, 237, 837?  Or 245, 445, or 645?  Of course IH 69 was just US 59 at the time so that's why that one wasn't considered.

[PColumbus73]

I'm not sure what the story with I-359 is, but between the through movements on the AL side and the exit numbers on the MS side, I-59 seems to be the primary route on the I-20/59 overlap.

I also suspect it's because I-359 runs north/south, and the state wanted a number that matched the direction it ran. This is the same state that originally signed I-565 as north/south originally, after all.

But there's also I-759 which is signed east-west.

I'm inclined to agree with Liliana that 3di's don't have much rhyme or reason, likely it's different in each state, too. It could also be something as simple as whatever rolls better off the tongue. Might also be worth noting that some are remnants of canceled projects, too.

In Ohio for example, it's interesting to me that they have no odd 3dis, so I would guess that Ohio wants their 3dis to connect end-to-end to other interstates. I-280 might have been signed to associate it with the Turnpike.

[hotdogPi]

While I haven't looked into which parent gets the last two digits, I have looked into whether a high or low first digit means anything. My hypothesis was that a lower first digit is longer, because of things like I-135 (KS), I-195 (MA/RI), and I-155 (IL), that are what I have called "medium-distance straight lines" in other threads.

I have found that this is not the case; the correlation is actually slightly negative with regard to my hypothesis (contrary to what I thought, lower is shorter, although it's statistically insignificant), partly because of things like Florida's I-175.

There's probably a correlation between first digit and creation date, but I haven't checked yet.

[Big John]

And the case of surrounding states with I-94? Low in Michigan (I-194) and Illinois (I-294), mid in Minnesota (I-394, 494, 694), high in Wisconsin (I-794, 894)

[kphoger]

Child routes is what happens when Interstates don't wear rubbers.

That would be tire-some.

By the way (etymology nerd here), did you know that rubber trees are named after erasers—and not the other way around?

1.  When you want to rub out a pencil mark, you reach for what Americans call an eraser but Brits call a rubber.

2.  The material those are made of came to be called by the same name:  rubber.

3.  The tree from which the material is sourced, then, also came to be called by the same name:  rubber.

[GaryV]

There are many instances of the first digit of 3di's (within a state) being assigned in numerical order. Low to high: S to N and W to E. When new 3di's pop up between existing ones, all bets are off.

States might often avoid duplicating a number in an adjacent state, if the duplicates were close enough together that it might cause confusion.

[kphoger]

The states submitted numbering to AASHO, so they got to choose. AASHO only got directly involved when routes crossed state lines, didn't follow numbering, or were otherwise janky (Iowa wanting an x35 and x80 overlapping through Des Moines).

I love how the actual correct answer seems to have gone unnoticed.