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[I-55]

Pretty sure in real life it's whatever route was built first.

There’s some states that would renumber exit numbers in a newly designated concurrency just to fit one of my 5 guidelines. Missouri is an example. The section of I-55 between I-44 and I-64 was originally I-55 only, and used I-55 exit numbers. After I-70 got rerouted and I-44 extended, this section’s exits got renumbered to follow I-44’s mileposts. Thought this made sense because I-55 leaves the concurrency on a 25 mph ramp to the PSB with I-44 as the through movement at the north/east end.

Not that I-55 leaving on a 25 mph ramp is a unique circumstance or anything...

[ran4sh]

Pretty sure in real life it's whatever route was built first.

There’s some states that would renumber exit numbers in a newly designated concurrency just to fit one of my 5 guidelines. Missouri is an example. The section of I-55 between I-44 and I-64 was originally I-55 only, and used I-55 exit numbers. After I-70 got rerouted and I-44 extended, this section’s exits got renumbered to follow I-44’s mileposts. Thought this made sense because I-55 leaves the concurrency on a 25 mph ramp to the PSB with I-44 as the through movement at the north/east end.

Or because the section north of I-64 is I-44 only, they decided it made logical sense for the I-44 numbering to be continuous.

[hbelkins]

The I-75/I-64 concurrency is interesting, because there's not very much difference between the mileage of the two in the state -- a mile or two at best.

I remember when the KY 1747 Hurstbourne Lane interchange was the easternmost Louisville exit with any significance. The Blankenbaker exit hadn't yet been built, and the Jefferson Freeway (KY 841, now Gene Snyder Freeway with I-265 added) extended only between US 60 and KY 155. There was an "Ashland 180" mileage sign just east of the Hurstbourne interchange.

[SkyPesos]

The I-75/I-64 concurrency is interesting, because there's not very much difference between the mileage of the two in the state -- a mile or two at best.

The in-state distance for those two are very close. Both have an exit 191, and I-75 also have an exit 192 in the 191 milepost, but I-64's exit 191 seems to be farther from the border than I-75's exit 191. I thought I-64 was a bit longer than I-75 for a while because of that, but it turns out that I-75 is 0.7 miles longer than I-64.

[PurdueBill]

I thought that “lower number gets priority” was a rule written down someplace.

In any case it lends itself to what I think is the worst numbered concurrency: I-15 and I-90 in Butte, MT. I-90 is the dominant route at both ends of the concurrency, both in terms of traffic counts and interchange configuration...but the overlap uses I-15 exit numbering, and the spur into Butte is designated I-115.

Holy cow that's an egregious flaunting of common sense, if you showed me that with no prior knowledge, I may have bet a thousand bucks that 90 gets the exit numbers and the spur.

I would too. Literally everything in there points towards I-90 having the exit numbers
1) I-15 leaves the concurrency on a single lane trumpet on both sides, while I-90 is the through movement.
2) I-90 is a much longer route in the state than I-15 (554 mi vs 396 mi)
3) I-90 mileage during the concurrency is higher than I-15 (219-227 vs 121-129)

The worst part of the I-90/I-15 boobery is that in both directions of the breakup, it is signed as an exit to stay on I-15, using the I-15 exit number, which makes absolutely no sense.  It should either have I-90 as the "exit" (even though 15 is taking the 1-lane trumpet ramp and 90 is 2 lanes straight), or display no number (ala I-76 WB and I-77 NB in Akron), but not have what it has.  It's evidence that they know 15 is the exit really.

15 sags south as it heads NB in order to hitch onto 90, showing that 90 is the boss in the pair.  Seems like engineering judgement would be appropriate vs. a fixed rule of lowest number wins.

[ethanhopkin14]

Texas' two interstate concurrencies are both in San Antonio and are both with I-35, and both use I-35 mileage.  The I-35/I-410 makes since because I-35 shouldn't be using I-410's exists in the 30s over I-35's in the 160s, but the I-10/I-35 concurrency, I always thought I-10 should get the mileage since it was the more dominate route.  The reason I-35 gets the numbering is because I-10 is clearly running on I-35's roadway and not the other way around. 

[sprjus4]

^

I-35 seems to make the most sense given it's the continuous route. I-10 is clearly using its roadway to connect to the other segment of I-10 northwest of Downtown.

I-37 / I-69E would be the next interstate concurrency, which it seems like I-37 would logically keep its exit numbers given it was first, and that I-69E is clearly running on its roadway, not the other way around.

[frankenroad]

I'm not sure if there's already a guideline for exit number priority in interstate-interstate concurrencies in the MUTCD, but I'm guessing there isn't from all the inconsistencies I found in various examples around the country. I decided to create my own unofficial guidelines that makes the most sense to me, and is what I use in my fictional exit lists.

1) In concurrencies with a 2di and 3di, the 2di would have exit number priority: Examples: I-74 with I-275, I-95 with I-495, I-29 with I-435.
2) In concurrencies where one route clearly exits off at both ends of the concurrency, the through route would have exit number priority. Examples: I-75 with I-71, I-70 with I-71, I-76 with I-70, I-15 with I-80.
3) In concurrencies where one route clearly exits off on one end, but the other end isn't clear on which route looks like the exit because of lane count, angle of exit, left side placement, etc, the through route through the definite exit would have exit number priority. Examples: I-65 with I-70, I-75 with I-85, I-77 with I-64.
4) In concurrencies where one route exits off at one point, and the other exits off at the other point, the longer route in the state would have exit number priority. Examples: I-57 with I-70, I-75 with I-64. This is what I went off with in a fictional idea that involves an I-70 and I-95 concurrency.
5) In concurrencies where the exiting route can't be determined because of lane count, angle of exit, left side placement, etc, on both ends, the longer route in the state would have exit number priority.

Of course there are a couple of "rule breakers" examples around, like the I-90/I-94 concurrency in IL should be using I-90's exit numbers based on my guidelines. Part of the rule breakers are for historical reasons, like in the I-40/I-85 concurrency, that route was solely I-85 when it was first designated, and I-40 was added on later.

Would be interesting to see if anyone else have their own guidelines of sorts for exit number priority in interstate-interstate concurrencies, or any other exceptions to my guidelines.

I believe, in most cases, that a route number ending in 0 or 5 trumps routes ending in 1,2,3,4,6,7,8, or 9.

[SkyPesos]

I believe, in most cases, that a route number ending in 0 or 5 trumps routes ending in 1,2,3,4,6,7,8, or 9.

Probably depends on the situation. Some examples of another digit over an x0/x5 are obvious why, like I-76 over I-70 in PA. I-59 over I-20 in MS was mentioned earlier in the thread. Illinois is the worst offender if x0 or x5 exit numbers should take priority over the other digits, with I-57 over I-70 and I-94 over I-90. I already mentioned why I-90 should have exit number priority over I-94 in the op, and the I-57 over I-70 makes sense, with the former route having higher exit numbers in the state.

And then there's another situation that is interesting; toll road mileage over an interstate's mileage. I know why the NY Thruway does it; to not have duplicated exit numbers on the toll road. But for some other examples, like the Kansas turnpike over I-70's, why? I don't think the Kansas Turnpike would have exit numbers between 366 and 410 by itself, so duplication is not a reason for the turnpike.

[kphoger]

And then there's another situation that is interesting; toll road mileage over an interstate's mileage. I know why the NY Thruway does it; to not have duplicated exit numbers on the toll road. But for some other examples, like the Kansas turnpike over I-70's, why? I don't think the Kansas Turnpike would have exit numbers between 366 and 410 by itself, so duplication is not a reason for the turnpike.

That's because you see the Turnpike as something that overlays I-70.  If you saw I-70 as something that overlays the Turnpike, then your opinion might be different.