The Mechanics of Branch/Child Routes

[Amaury]

So, I think I finally have a good understanding of how child routes work, at least here in Washington State. I'm not sure if it's the same in other states. I've been wanting to expand more in my fictional highways thread, but I wanted to have a better understanding on child routes first.

For example, I-5 gets 5xx for child routes, such as SR 505, US 12 gets 12x for child routes, such as SR 124, SR 26 gets 26x for child routes, such as SR 260, and so on. Interstates, US routes, and state routes can all have child routes, but interstates and US routes cannot be child routes, if I understand correctly. For example, SR 26 couldn't have a hypothetical I-265 or US 265 as child routes. Highways like US 95 and US 97 also have x95 and x97, such as US 395 and US 197, but those are auxiliary routes, not child routes, so they work differently.

This leads me to a question. My other understanding is that child routes must connect to the parent route. So, how is it decided what a child route's parent route will be when it connects to two separate parent routes? For example, SR 211 connects SR 20 to US 2, with SR 20 being one of the exceptions to the standard, as it uses 21x for its child routes since US 2 already uses 20x, and from what I can see, SR 21 has no child routes. US 101 is another example, using 10x for its child routes since SR 10 has no child routes. Similarly, SR 9 uses 9x for its child routes since 90x is already in use by I-90.

Anyway, SR 211 is a child route of SR 20, but it could just as easily be SR 201 since it also connects to US 2, so why did they choose SR 20 as its parent route? Just like in concurrencies with different route types, where the higher-level route is the primary route (Interstate -> US route -> state route), such as US 97 being the primary route in the US 97/SR 20 concurrency, you'd think the same would apply here. And SR 201 is not taken. Similarly, SR 281 is a child route of SR 28, but also connects to I-90, which uses 90x for its child routes, and could just as easily be SR 909?

My second question regards US 97. US 97 gets 97x for its child routes in Washington State, such as SR 970 and SR 971. SR 150, SR 153, and SR 155 all appear to be child routes of US 97, but their numbers indicate being child routes of SR 15, even though SR 15 does not exist in Washington State. So, shouldn't they be SR 970, SR 973, and SR 975, respectively? Obviously, SR 970 is already in use in Kittitas County, so that one probably should have been something like SR 908 or SR 910 to begin with, a child route of I-90 instead of a child route of US 97 and then the current SR 970 designation could have gone to the current SR 150 in Chelan County, but I don't want to fictionalize this too much.

My last question regards SR 22, also in Washington State. It gets 22x for child routes, which are SR 221, SR 223, SR 224, and SR 225. However, SR 224 and SR 225 don't ever touch SR 22, so why weren't they numbered as child routes of I-82, which they do touch? SR 824 and SR 825, respectively. I guess in very technical terms, SR 224 doesn't touch I-82, as it ends at the junction with SR 225, and doesn't touch the on and off ramps, but it's a lot closer to I-82, as it is literally right there at Exit 96. It is nowhere near SR 22. SR 223, which does touch SR 22, would probably work better as SR 224, since, despite the odd number and north-south directionality, it is definitely an east-west highway.

On a final note, SR 397, which is described as an auxiliary route of US 395 on Wikipedia, would probably fit better as SR 827, a child route of I-82. US 395 itself is already an auxiliary route of US 95. Can auxiliary routes have their own auxiliary routes? Again, though, I don't want to fictionalize this post too much, so I will just leave it there.

EDIT: I meant to post this in General Highway Talk, if someone could move it there, please.

[Bitmapped]

For Interstates and US routes, the parent route makes up the final digits of the child routes. For example, allowed child routes of I-5 would be Interstates 105, 205, 305, 405, 505, 605, 705, 805, and 905.

Different states have their own patterns for numbering. Pennsylvania, which has explicit parent/child relationships, uses the same pattern as Interstates and US routes. Virginia once used appended numbers for child routes as you're describing, but that was back in the 1920s before later mass renumberings.

[vdeane]

In New York the child routes have letter suffixes of the route they branch from.  Usually they're "loops"/bypasses but sometimes spurs (see: NY 12F).

In West Virginia, this is what the fractional routes are.

[LilianaUwU]

In Québec, children of Autoroutes only run from 4xx to 9xx to avoid conflict with the 100-399 routes, but are otherwise similarly numbered like Interstates. Regular highways don't have children, and Autoroutes don't have routes as children, though numbering coincidences exist in plenty of places (A-55/QC 155, A-73/QC 173, QC 143/QC 243, etc.)

[vdeane]

though numbering coincidences exist in plenty of places (A-55/QC 155, A-73/QC 173, QC 143/QC 243, etc.)

Aren't those a consequence of Québec's grid system?

[LilianaUwU]

though numbering coincidences exist in plenty of places (A-55/QC 155, A-73/QC 173, QC 143/QC 243, etc.)

Aren't those a consequence of Québec's grid system?

Yeah. It does make for a fairly clean system, though.

[ilpt4u]

IL 38 used to be designated as US 30A. Imagine a Chicago accent and say both out loud. IL 38 is a child of US 30.

[Henry]

IL 38 used to be designated as US 30A. Imagine a Chicago accent and say both out loud. IL 38 is a child of US 30.

To me, that could also be considered a route numbering coincidence, as "38" does sound very similar to "30A".

[bzakharin]

What's the difference between child routes and auxiliary routes? In New Jersey 3-digit state routes are either downgraded Interstates (NJ 495 used to be I-495), former US routes (NJ 109 is the former alignment of US 9), former alignments of state routes (NJ 138 was to connect to NJ 38, but was later canceled), auxiliary state routes (NJ 347 is a bypass of part of NJ 47), or connects to an out of state route with the same number (NJ 440 connects with NY 440).

[ilpt4u]

IL 38 used to be designated as US 30A. Imagine a Chicago accent and say both out loud. IL 38 is a child of US 30.

To me, that could also be considered a route numbering coincidence, as "38" does sound very similar to "30A".

Especially with the Chicago accent

[epzik8]

I don't believe the notion of child state routes exists in Maryland, but each county does have at least one allotted cluster of state routes (two in some of the more populated counties) from 38 to almost 400. No county has a mainline serving as a parent of any of said routes, though.

[Bitmapped]

I don't believe the notion of child state routes exists in Maryland, but each county does have at least one allotted cluster of state routes (two in some of the more populated counties) from 38 to almost 400. No county has a mainline serving as a parent of any of said routes, though.

Garrett County has MD 742, the pre-I-68 alignment of MD 42 through Friendsville. MD 946 is the pre-I-68 alignment of MD 546.

Maryland also has a lot of letter-suffixed unmarked routes that are tied to the parent route they branch off. Sometimes, it's a suffix on the signed parent route (like US 219J or MD 42A). Other times, there are suffixes off an unsigned child route number - for example, MD 826A/B/C/etc are all associated with US 219.

[epzik8]

I don't believe the notion of child state routes exists in Maryland, but each county does have at least one allotted cluster of state routes (two in some of the more populated counties) from 38 to almost 400. No county has a mainline serving as a parent of any of said routes, though.

Garrett County has MD 742, the pre-I-68 alignment of MD 42 through Friendsville. MD 946 is the pre-I-68 alignment of MD 546.

Maryland also has a lot of letter-suffixed unmarked routes that are tied to the parent route they branch off. Sometimes, it's a suffix on the signed parent route (like US 219J or MD 42A). Other times, there are suffixes off an unsigned child route number - for example, MD 826A/B/C/etc are all associated with US 219.

Yeah, I know all about the letter suffixes, but I guess they didn't cross my mind because they're literally unmarked, that's how subtle they are.

[Bickendan]

Washington is fairly unique in how it approaches its numbering, and it's actually very logical overall, with a few quirks that might be remnants of renumbering (such as WA 505 being east-west or north-south as discussed in another thread). Some oddities include WA 121, ostensibly a child of US 12, not connecting to US 12 at all, and WA 116 being a child route of US 101 but even less related than 121 is to 12 (100-119 are all US 101's child routes, much like I-5 having 500-548).

Oddities: WA 108, child of US 101 or WA 8 (general sense, not strictly following the rule of a child route of WA 8 being a 80x)? It connects to both.

I-705: An I-5 child (per interstate numbering) or a WA 7 child (per WA 70x numbering)? 705 directly extends north from WA 7's northern terminus *at* I-5...
Same logic can't be applied to I-182 and WA 18, however, as there's no relation whatsoever between the two.

[Dirt Roads]

Perhaps the most iconic of all parent/child route numbering systems is West Virginia's infamous "fractional routes", which are part of the misnamed County Road system (all of which are maintained by WVDOH).  Simple enough to understand, the parent's number is the numerator and the branch number is the denominator (for you math buffs, this probably seems backwards).

One of the surprises of the "fractional routes" is that WVDOH avoids routes with both 3-digit numerators and 3-digit denominators.  This is a problem for areas with lots of branches off of 3-digit routes such as US-119, particularly in counties where the entire route has been relocated.  For instance, in Logan County, the "fractionals" run from CR-119/1 to CR-119/99, then start a new sequence beginning with CR-219/1.  Curiously, the next set of children of US-119 start with CR-700/1, CR-701/1 and CR-702/1. 

In days gone by, there were "fractional routes" with 3-digit denominators, particularly in large counties with long routes.  Nowadays, the denominators appear to be limited to 2 digits.  The same approach above seems to be used.  For instance, Kanawha County has children of US-60 starting with CR-160/1 and children of WV-61 starting with CR-161/1.

[Kulerage]

North Carolina's state highways have an interesting relationship with this sort of thing. Originally, the primary highways were all two digit numbers ending in 0 (so 10, 20, 30, etc.), and then other highways were numbered as spurs off of the main highways, so NC 11 would've been the first spur of NC 10, then NC 12, and so on.Then, there were three digit state highways which were basically spurs of the spur highways, and were also numbered by appending the new digit at the back rather than the front. So for example, the first child highway of NC 12 wouldn't be NC 112 but rather NC 121. In other words, North Carolina originally had two levels of child routes!

However, North Carolina has renumbered its state highways about four different times, which has caused the original numbering system to be completely disregarded, and routes that still follow the original numbering plan are a bit hard to come by, to the point where I have no idea what determines a highway's number anymore.

[NWI_Irish96]

In Indiana, child routes are three-digit routes with the same last two numbers as the parent, which can be a state or US highway. Sometimes they connect to the parent, but more often they closely parallel the route. There is no odd/even first digit meaning like there are with interstates.

A few notable quirks:

IN 135 used to be IN 35 before US 35 existed, but then got changed to 135 to avoid US/state route duplication. IN 235 and 335 are thus children of IN 135, not US 35.

IN 134 and 234 are orphans, as IN 34 was replaced by US 136 and no longer exists. Similarly, IN 121 was orphaned when IN 21 became US 35.

IN 129 and 229 appear to have no relation to the far away IN 29, but before the advent of US 421, IN 29 ran all the way down to the Ohio River.

[Bitmapped]

One of the surprises of the "fractional routes" is that WVDOH avoids routes with both 3-digit numerators and 3-digit denominators.  This is a problem for areas with lots of branches off of 3-digit routes such as US-119, particularly in counties where the entire route has been relocated.  For instance, in Logan County, the "fractionals" run from CR-119/1 to CR-119/99, then start a new sequence beginning with CR-219/1.  Curiously, the next set of children of US-119 start with CR-700/1, CR-701/1 and CR-702/1. 

It's not so much that they avoid 3-digit numerators just as relatively few cases where there would be any. For state routes, WV has fairly few 3-digit numbers and they're fairly recent, meaning that most of any children are still numbered for the previous route that was there. In the cases where it is a 3-digit WV route or US route, you'll see 3-digit numerators on fractional county routes.

Only one county, Preston, has regularly numbered county routes into the triple digits. Preston's go up to CR 120 in the southeastern corner of the county. All of the other instances of 3-digit county routes were specially numbered as they were.

The 700-series county route numbers in Logan County seem to be unique to Logan County. The routes are scattered throughout the county, not tied to any particular parent. I suspect they were either former delta or HARP routes that were added to the county route system in bulk at some point.

In days gone by, there were "fractional routes" with 3-digit denominators, particularly in large counties with long routes.  Nowadays, the denominators appear to be limited to 2 digits.  The same approach above seems to be used.  For instance, Kanawha County has children of US-60 starting with CR-160/1 and children of WV-61 starting with CR-161/1.

WVDOH's internal route ID scheme, which folds county number, system type, numerator, and denominator into one, only allows for 2 digits for the denominator. That's why 99 is the highest denominator you'll see nowadays and they bump up the numerator by 100 and reset the denominator if they need to continue on.

[DandyDan]

Minnesota doesn't do child routes, generally speaking, but MN 149 is a former route of the previously existent MN 49 and MN 252 was a previous route of US 52. I believe MN 152, when it existed, was also a previous route for US 52.  MN 610 is a spur of US 10.

Iowa does child routes even less, but IA 471 is a previous route for US 71.

[TheStranger]

California doesn't have that many examples of this:

HISTORICAL
- Route 740 and Route 440, short-lived connections to Route 74 and Route 44 in the creation of the signed state routes in 1934
- Route 107, a former routing of Route 7 in Inglewood/Hawthorne near LAX starting in the early 1940s.  As that Route 7 was decommissioned in favor of I-405 in 1958, 107 has now outlasted its nominal parent for a full 66 years and counting, even though it has been shortened from its original extent (as the portion going through Inglewood was given to local control by the 1970s).

CURRENT
- Route 371, a 1970s designation for a former portion of State Route 71 that was orphaned when I-15 was established on the Devore-Ontario-Corona-Temecula corridor and moved off of old US 395.
- Route 330, a 1970s designation for a former portion of State Route 30 created when then-30 (now 210) was rerouted south of Highland to connect to I-10.
- Route 242, a 1960s designation for what was built and signed as Route 24 through Concord (on a routing that had existed since the 1930s-1940s), in anticipation of a Route 24 bypass along Willow Pass Road east of Walnut Creek that never got built.  Finally signed under its actual number and given north-south cardinal directions in the late 1980s.

[jp the roadgeek]

I can only think of a few examples of child routes in CT, mostly as an old routing of an existing route.  The best example is CT 372, which was born when CT 72 was rerouted onto an expressway, then extended over its former parent's route when CT 72 was truncated to Nee Britain when the CT 9 extension north of I-91 was completed.  CT 272 is a remnant of when CT 72 used to extend all the way to the MA border in Norfolk.  However, CT 172 is no relation.  The only other example I can think of is CT 334 as a child of CT 34.  However, they do not connect.  There are a couple in SR's, such as SR 571 near CT 71 and SR 695 as an offshoot of I-395.

[kurumi]

I can only think of a few examples of child routes in CT, mostly as an old routing of an existing route.  The best example is CT 372, which was born when CT 72 was rerouted onto an expressway, then extended over its former parent's route when CT 72 was truncated to Nee Britain when the CT 9 extension north of I-91 was completed.  CT 272 is a remnant of when CT 72 used to extend all the way to the MA border in Norfolk.  However, CT 172 is no relation.  The only other example I can think of is CT 334 as a child of CT 34.  However, they do not connect.  There are a couple in SR's, such as SR 571 near CT 71 and SR 695 as an offshoot of I-395.

Also:
* 289 was old 89
* If I-684 had stayed I-87, CT was prepared to renumber CT 87 to 287 (this was before the real CT 287 existed)
* 209 was part of 109

CT also did a lot of this when promoting former "secret" routes to signed; almost any signed route above 220 (example: 222 was 722) matches a previous SR or route. The list of counterexamples is shorter:
* 319 (was 611, but intersects 19; this might be the only deliberate "designate a child route" that's not a former route)
* 234 (was 645)
* 337 (was 704)
* 244 (was 622, but close to US 44)
* 286 (was 802 in part? 286 history is murky)
* 287 (was 544)

For all except SR 622, "rhyming" numbers were available (311, 245, 304, 302 at the time, and 344) but not chosen.

[Amaury]

So, I have a couple of questions, at least regarding to child routes in Washington. Actually, I asked these in the OP, but I realize, I made a big post.

1) When a child route connects to two parent routes, how is it decided which parent the child belongs to? For example, WA 211, a child route of WA 20, connects to both US 2 and WA 20. So, why did they decide to number it WA 211 and not something like WA 201, a child route of US 2? Just like the higher route type is the primary route in a concurrency, such as I-82 -> US 97 or US 97 -> WA 20, I'm wondering why the same doesn't follow for parent and child routes, where the higher route type is a child's parent.

2) Why are WA 224 and WA 225 child routes of WA 22? They never intersect WA 22. Slightly fictional territory here, but it seems like it would be more appropriate for them to be child routes of I-82, such as WA 824 and WA 825.

[Bickendan]

211 looks to be a child route of US 2, not WA 20.

I'd guess proximity to WA 22's general area for 224 and 225, though US 12 would probably be a better parent route choice (even over I-82).

[Amaury]

WA 20 uses 21x for its child routes since US 2 already uses 20x for its child routes, and WA 21 has no child routes. And if I remember correctly, single digit numbers only have nine numbers available for child routes, so US 2 only has 201–209 available for child routes. Unless WA 211 is like a super rare exception since I realize these are just guidelines and not top-down rules, which is why we also have things like I-82 being north of I-84, which some people call an abomination.

For WA 224 and WA 225, I chose those since everything for US 12 (12x) is practically taken. The only ones available that I can see are WA 120 and WA 126, which are no longer in use and are both evens, though I suppose one of them could be one of those that is an even number, but is signed as north-south, like WA 548.