Picking parents for 3dis

[Mr_Northside]

Another unusual example that I realized today: Why is I-576 in Pittsburgh an x76 instead of an x79? Yes, I know it meets I-376, but it doesn't meet I-76 itself, while it does meet I-79. Shouldn't that alone make an x79 number better than an x76? Also, it's north-south, which parallels with I-79 more.

My best guess would be that Pennsylvanians tend to associate 76 with toll roads, so 576 fits right in with 276, 376, and 476, all of whom are tolled or have tolled segments. I-79 is free for its entirety as well.

At first, I thought about the fact that PA-TPK-576 didn't go to I-79 for 15 years until just about a year and a half ago could factor in.  Though at the time the "Findlay Connector" opened in 2006, current I-376 near the airport was still PA-60.  Though I think it was pretty much a given at that point that it would be renumbered to I-376.
Also, way back in the day, I'd speculate (and only speculation) that when the Mon-Fayette was supposed to "Y" into a highway going toward downtown (since taken off the table) and the other leg to the Parkway East (I-376) in Monroreville, the 576 number would be concurrent with TPK-43 from the future interchange (if/when built) near the Washington/Allegheny County line to where they'd have split near the Monongahela River, with TPK-43 heading toward downtown in the cancelled highway, and 576 meeting back up with 376 in Monroeville (creating a true Southern Beltway, with 376 as both termini).

Or, maybe the people that actually get to choose the numbers didn't put quite as much thought into it as people here do.

[wanderer2575]

A variant of the topic is picking the parent for a 3di that has its applicable end(s) at a junction where two 2dis run concurrently -- for example, I-359 in Tuscaloosa, AL (spur with a terminus at I-20/I-59) or I-280 outside Toledo, OH (southern terminus at I-80/I-90). 

Also, I've seen concurrent US routes have concurrent business loops (WY); why couldn't concurrent 2dis have concurrent 3dis?

[MikeTheActuary]

I know it's been a couple of weeks, but....

How is I-270 incorrect? It connects with an interstate on either end.... I-70 (its parent to the north) and I-495 to south.

It must connect to the same interstate at both ends to make it a true loop. It should be a spur, hence numbered I-170 for example.

An auxiliary interstate does not need to be a loop to be eligible for an even first digit.    Even first digits are intended to communicate that it is a "connecting" auxiliary interstate.

I-270 in Maryland connects I-70 and I-495, and therefore is eligible for an even first digit.
I-291 in Connecticut connects I-84 and I-91 and therefore is eligible for an even first digit.

[ran4sh]

I believe it should be whether it's radial or circumferential. For example, I-291 in Connecticut is correct despite not meeting I-91 at both ends because it's a partial loop around Hartford, not inward or outward. On the other hand, I-270 in Maryland is incorrect because it clearly goes to and from DC.

"Radial" and "circumferential" quickly lose meaning when dealing with large urban areas with multiple dense nodes, such as Los Angeles (where many of the 3di are in grid layout anyway), or regional areas with multiple urban centers, such as the NC urban region containing Raleigh-Durham-Chapel Hill, as well as the one containing Greensboro--Winston-Salem--High Point.