Picking parents for 3dis

[hbelkins]

Are there guidelines for choosing which interstate gets to "parent" the 3di routes in a certain location?

It's not always the lowest number in a community. In Richmond, I-95 gets to be a parent while I-64 does not. However, in Louisville, I-64 was a parent long before the idea of numbering the Jefferson Gene Snyder Freeway as I-265 was floated. In Atlanta, I-20 is the lowest-numbered 2di, and I-85 is the highest-numbered, and I-85 gets to be the parent. In Birmingham, the middle number (59) is the parent.

[OCGuy81]

I've often wondered the same.

If I had to guess, and this is just that, a guess, they base it on when the routes were planned.  In the case of Atlanta, was the routing of I-85 established prior to 75 and 20?  I'm not sure, but that might be one factor.

[sprjus4]

I think for Richmond, it's because the route (I-295) is largely a bypass for I-95 north-south traffic, however the western ~12 mile portion does provide a full east-west bypass for I-64 (one I've used dozens of times) as well, but it's not nearly as long as the ~43 miles of bypass for I-95. The northeastern leg between I-95 and I-64 serves as a joint bypass for both I-64 and I-95 traffic.

As far as the others... VA-895 (planned I-895) is pretty obvious - it links I-95 and I-295. I-195 / VA-195 does link to I-64 on one end, but it also connects both ends to I-95, so that makes more sense.

I also think I-64 numbers in Virginia weren't used due to the heavy concentration / need for 3di numbers in the Hampton Roads region. VA-164, I-264, I-464, I-564, and I-664 are all in use there.

To answer the original question... I would just say it depends on the situation case-by-case.

[JoePCool14]

I would vote for it being case by case basis. Chicagoland's 3DIs are fairly clear as to which number makes the most sense. 294 is clearly an alternate to 94, 290 is clearly an alternate to 90, 190 is clearly a 90 spur, etc.

While not technically an interstate, IL-394 is probably the only one where you could've chosen an x80 number instead of an x94. But I think x94 is a better choice, given how many x80 routes there are across the country.

Milwaukee definitely had other options for 794, could've easily been an x43 number, say 143, and honestly, it's too bad it's not, since 43 has no child routes.

Overall, for bypasses, pick the route that the bypass is more effective or helpful for. For spurs, pick whatever you feel like.

[Big John]

Milwaukee definitely had other options for 794, could've easily been an x43 number, say 143, and honestly, it's too bad it's not, since 43 has no child routes.

That was established before I-43 existed.

[JoePCool14]

Milwaukee definitely had other options for 794, could've easily been an x43 number, say 143, and honestly, it's too bad it's not, since 43 has no child routes.

That was established before I-43 existed.

Ah okay, thanks.

Well in my ideal world, I'd call it I-143. (Bonus points: there currently is no WI-143, so it's available!)

[Big John]

Milwaukee definitely had other options for 794, could've easily been an x43 number, say 143, and honestly, it's too bad it's not, since 43 has no child routes.

That was established before I-43 existed.

Ah okay, thanks.

Well in my ideal world, I'd call it I-143. (Bonus points: there currently is no WI-143, so it's available!)

Decommissioned in 1996.

[Henry]

Los Angeles has some nice variety, with I-5 and I-10 being parents of three 3di's each. Even I-15 gets in on the action, although technically so, as San Bernardino may be considered the farthest east suburb.

OTOH, for Seattle/Tacoma, I-5 gets two children, but I-90 gets none.

[SkyPesos]

I wonder if beltways generally favor x0 and x5 3di numbers over non-x0 or x5, in addition to what has been mentioned already.
Like I-270 in Columbus could've easily been I-271 instead (it existed before both the existing I-271 and 471), as its north-south sections are longer than its east-west sections, and acts as an I-71 bypass as much as it is an I-70 bypass.

Also, a portion of St Louis's I-270 was I-244 before getting renumbered. Wonder if the x0 favoritism is a reason for it getting renumbered.

[achilles765]

I wonder this too. Like why is our loop in Houston interstate 610 and not interstate 245 or 445? Why is San Antonio's interstate 410 and not 435 or even 437 or 837?

[JoePCool14]

I wonder this too. Like why is our loop in Houston interstate 610 and not interstate 245 or 445? Why is San Antonio's interstate 410 and not 435 or even 437 or 837?

In those cases, 410 and 610 roll off the tongue better than 435 or 837. Human preference plays a part there. I find those examples more clear cut than 794 vs. 143.

[SkyPesos]

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.

[sprjus4]

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.

I-576?

[TheStranger]

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.

I-576?

I think he actually means what is I-579 (the short spur to Pittsburgh's arena district).  It had been proposed at one point as I-876 (was there a planned connection to I-376/former I-70/I-76 that was never constructed?)

[TheStranger]

California generally for the most part has logical 3di numbers.  The earliest examples were sequential, but several are derived from past designations of roads.

I-80
180 skipped due to 1934-present Route 180, though I-180 was a temporary designation for former Route 17/post-1984 I-580 extension between San Rafael and Albany
I-280 - partial beltway of SF-San Jose area first proposed in 1950s, oriiginally with a planned terminus at I-480 in the Presidio (today's US 101/Route 1 junction), then a planned but unconstructed terminus at I-80 near the Bay Bridge.  Of note: the original 1950s-1960s plans for I-80 would have had 80 end at 280 in Golden Gate Park
I-380 - spur of I-280 to San Francisco International Airport, a replacement number for what had been planned as Route 186.  Designated after I-580 came into existence in the late 1960s/early 1970s.
I-480 - was planned in the 1950s as an inner loop around the Marina and Embarcadero (partially built, now demolished) from the foot of the Bay Bridge to today's US 101/Route 1 junction in the Presidio (where Route 1 south of there had been proposed as the northernmost segment of 280 pre-1968)
I-580 - former I-5W between I-80 and I-5, designated 1964
I-680 - east half of Bay Area belt route proposed in 1950s, from San Jose through Concord originally ending in Vallejo, then rerouted in 1976 to Cordelia.
I-780 - former I-680 between Benicia and Vallejo, designated 1976
I-880 (original) - bypass route of old I-80 in West Sacramento and Sacramento. Taken over by I-80 in 1982 as a result of the through-town routing upgrades in North Sacramento being canceled in 1979 in favor of light rail.
I-880 (current) - former Route 17 between the MacArthur Maze and I-280 in San Jose, existed as freeway for 20-30 years before Interstate designation was created in 1984.  Not really a loop route but road does link two interstate (even though freeway segment of the remaining Route 17 does continue past I-280 and Route 85)
I-980 - planned Route 24 extension from I-580 west to what is now I-880, also part of a corridor that had been in some proposals for the Southern Crossing project.  At the time route was designated (1981), the 480 number still existed in SF, and I-880 along the Nimitz was about 3 years away from happening so this essentially was "spur from I-580 to a non-interstate road".
I-238 - lol

I-5
I-105 (original) - existing US 101 between the East Los Angeles Interchange and the San Bernardino Split.  Never signed.
I-105 (current) - connector from LAX/Route 1 east towards I-605 and near, but not at I-5.  Spur from I-605 essentially to the airport area and El Segundo.
I-205 - connector between I-580 in Altamont and I-5 in Tracy, essentially a side effect of changes in I-5 (creation of West Side corridor instead of US 99 becoming I-5 here + I-5W not going to Route 132 in Modesto) ca. 1957-1958.  Former US 50.
I-305 (unsigned) - funding designation from 1982 onwards for former I-80 in West Sacramento and Sacramento, essentially the Interstate-standard portion along US 50 from I-80 to Route 99, and Route 51 from Route 99 to E Street. Designation created after I-505 was created.
I-405 - west loop of I-5 through West LA, LAX, Long Beach, Costa Mesa.  IIRC this existed before I-205 was a thing?  Planned in the 1950s along former Route 7 (Sepulveda) as well as a new-terrain corridor from Hawthorne southeast to Irvine.
I-505 - former planned I-5W between Dunnigan and Vacaville, designation is from 1964.
I-605 - connector originally designated between I-405 in Seal Beach and I-10 in El Monte, later extended northeast to I-210.  Proposed in the 1950s
I-805 - east bypass of I-5 in San Diego, proposed 1950s (surprising that this wasn't I-205 at the start in retrospect).
I-905 (currently signed as state route) - former Route 75/Route 117 from I-5 southeast to the Otay Mesa border, designation created in 1980s but Interstate signage still has not begun

I-10
I-110 (original) - today's unsigned Route 10 connector along former US 60/70/99 between US 101 and I-5.  Never signed.
I-110 (current) - former Route 11/US 6 between US 101 (where southbound signage with the interstate shield begins in earnest) passing by I-10 (where the interstate designation itself actually starts?) to Route 47 in San Pedro.  Designation appears to be derived from the former Route 11 numbering and was created in 1981.
I-210 - northern loop of I-10 from San Fernando throug Pasadena originally to Pomona (State Route 210 later extended this loop route functionally to Redlands, and was submitted in the late 90s as possible Interstate but not approved at the time).  Proposed in the 1950s.
I-710 - former Route 15/Route 7 along the Long Beach Freeway from I-10/Valley Boulevard near Monterey Park to Route 47 on Terminal Island. Designation created 1984 (at the same time as 880) and is very much derived from the 1964-1983 Route 7 numbering.

I-15
I-215 - lengthy urban loop that is former US 395/I-15/I-15E between Devore and Temecula, passing through San Bernardino and Riverside.  Designation created 1982 to replace 1974-1982 I-15E, which itself was created after I-15 was realigned to a new-terrain bypass route from Devore to Temecula that incorporated Route 71/former US 395 and planned Route 31. (note that all of this was still US 395 prior to 1969)

[74/171FAN]

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.

I-576?

I think he actually means what is I-579 (the short spur to Pittsburgh's arena district).  It had been proposed at one point as I-876 (was there a planned connection to I-376/former I-70/I-76 that was never constructed?)

SkyPesos is referring to PA 576 (AKA PA Turnpike 576).

[Scott5114]

I wonder this too. Like why is our loop in Houston interstate 610 and not interstate 245 or 445? Why is San Antonio's interstate 410 and not 435 or even 437 or 837?

In those cases, 410 and 610 roll off the tongue better than 435 or 837. Human preference plays a part there.

I've never noticed anyone in Kansas City having any problem rolling 435 off their tongue.

[SkyPesos]

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.

I-576?

Sorry, I meant PA 576. Forgot that it wasn't an interstate, but a state route signed like a 3di.

[SkyPesos]

I wonder this too. Like why is our loop in Houston interstate 610 and not interstate 245 or 445? Why is San Antonio’s interstate 410 and not 435 or even 437 or 837?

In those cases, 410 and 610 roll off the tongue better than 435 or 837. Human preference plays a part there.

I've never noticed anyone in Kansas City having any problem rolling 435 off their tongue.

Yep, I've never seen a problem with the other notable xx5 beltways "not rolling off their tongue" over an xx0 (I-465, I-275, I-495 to name a few)

[Quillz]

I've never had an issue with any given number not rolling off the tongue. Some numbers are more fun to say (Route 66, for example). None of them are hard to say and I don't think that was really reasoning for why a number was or wasn't chosen. But it's an interesting idea at the very least. But I also think retaining existing numbers is more important than renumbering something to a 3di, especially nowadays. In the early days this probably made sense as a way to promote the system, but now the design standards and funding are more important. (And I don't think the latter is even a thing anymore). In the case of California, if they could do it over, I'd say just don't even bother with I-238. Just leave it as CA-238, or just keep the whole thing as CA-9 (which it originally was anyway).

What I find interesting is states that have few 3di, not always choosing the lowest available. Like in Nevada, it was I-515 as opposed to either I-115 or I-315. On one hand you could argue this was perhaps due to planned future expansion, but Las Vegas is in the southeast corner of the state, the odds of either a 115 or 315 showing up farther south seems pretty slim. The odd number makes sense given odd leading digits are usually used as spurs, but I've always wondered where that number came from.

[SkyPesos]

What I find interesting is states that have few 3di, not always choosing the lowest available. Like in Nevada, it was I-515 as opposed to either I-115 or I-315. On one hand you could argue this was perhaps due to planned future expansion, but Las Vegas is in the southeast corner of the state, the odds of either a 115 or 315 showing up farther south seems pretty slim. The odd number makes sense given odd leading digits are usually used as spurs, but I've always wondered where that number came from.

Some states do that to not duplicate with 3di in adjacent states. From what I heard, Milwaukee went with 894 because Chicago had 294 already, and 494 and 694 planned. Similar reason for 794: 194 was on the Kennedy Expy when I-90 used the alignment of existing 290, 394 is a state route 3di, and 594 was the O'hare spur.

Also, Charlotte went with 485 for their beltway because Atlanta already had a well-known 285 beltway.

[MATraveler128]

I never understood why North Carolina went for I-795 rather than I-195, 395, or 595. Maybe for duplication purposes? I can sort of understand why Georgia went for I-985 as the spur to Gainesville. I-185 is taken already in Columbus and I guess they thought that I-385 and I-585 in South Carolina were too close, but 785 was available.

[TheStranger]

In the case of California, if they could do it over, I'd say just don't even bother with I-238. Just leave it as CA-238, or just keep the whole thing as CA-9 (which it originally was anyway).

Route 9 north of Los Gatos was already truncated by 1964; the segments between 238 and the current Route 9 terminus were covered by former and current Route 85, Route 237, and Route 262.  (if anything, the San Jose-area Route 9 that existed pre-1964 was a hodgepodge corridor that represents several distinct corridors, as seen above).

[sprjus4]

I never understood why North Carolina went for I-795 rather than I-195, 395, or 595. Maybe for duplication purposes?

Probably not I-395 due to only being 200 miles away from Virginia / DC's I-395. I'm not sure why not I-195 or I-595.

Also - the same goes for NC I-587. Why not I-187, I-387, I-787, I-987? None of those exist anywhere, except I-787 near Albany, NY - and that's a separate I-87 system altogether.

Then again, they went with NC I-295 around Fayetteville instead of I-495 or I-695, despite being only around 150 miles from VA I-295. I know I-495 was the designation temporarily used on I-87 (when it was only for Raleigh to I-95), but I believe that came later than the I-295 idea.

[SkyPesos]

Also - the same goes for NC I-587. Why not I-187, I-387, I-787, I-987? None of those exist anywhere, except I-787 near Albany, NY - and that's a separate I-87 system altogether.

There's an I-587 in Kingston NY, though it's a pretty short glorified ramp that ends at roundabouts on both sides. Imo it's fine as just NY 28.