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[SkyPesos]

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.

[Revive 755]

The 2009 MUTCD does not specify which interstate gets priority.

Where numbered routes overlap, continuity of interchange numbering shall be established for only one of the routes (see Figure 2E-21). If one of the routes is an Interstate and the other route is not an Interstate, the Interstate route shall maintain continuity of interchange numbering.

As for exceptions to the rules in your post, the first rule has a few:

1) I-465 gets priority over I-74 around Indianapolis
2) I-694 gets priority over I-94 north of Minneapolis.
3) I-294 gets priority over I-80 south of Chicago

[I-55]

Your rule number 4 is the one I can think of the most examples of off the top of my head:

I-35/80
I-20/59 MS
I-40/65
I-40/24
I-40/75
I-40/55

you could also apply this to routes that have the higher mileage at the point of the concurrency. Including the examples above, I also know that the I-29/80 concurrency uses I-29's mileage despite it breaking all of the given rules.

Other exceptions for rule #1:

I-75/640
I-83/695
I-87/440 (historic reasons)

[ran4sh]

Your rule 4 probably has a higher priority in practice. For example, I-20/59 in Mississippi, the only logical choice is to use I-59 because its mileage is higher at the point they meet. If I-20's miles were used then I-59 would possibly have overlapping exit numbers. Similarly, I-75/85 is almost certainly because I-75's mileage at that point is higher than anything I-85 reaches on its own in Georgia. And not because of interchange design.

As for I-40/85, at least on the Greensboro end, the interchange is designed with I-40 as the exit. This is because they had intended that I-40 would continue with I-85 to the interchange south of Greensboro, and the interchange where they split now, was intended as the exit for Business 40 & Business 85.

[Scott5114]

In Oklahoma, Interstate-Interstate concurrencies use the lower-numbered Interstate's exit numbers. Which is a fancy way of saying that in both instances of this occurring in the state, I-35's exit numbers are carried through. This means that I-40, which has the longest Interstate mileage in the state, has two exit 127s (one at Radio Road west of the OKC metro, and again at M.L. King/Eastern Ave in OKC, along the I-35 concurrency).

[sprjus4]

Your rule 4 probably has a higher priority in practice. For example, I-20/59 in Mississippi, the only logical choice is to use I-59 because its mileage is higher at the point they meet. If I-20's miles were used then I-59 would possibly have overlapping exit numbers. Similarly, I-75/85 is almost certainly because I-75's mileage at that point is higher than anything I-85 reaches on its own in Georgia. And not because of interchange design.

As for I-40/85, at least on the Greensboro end, the interchange is designed with I-40 as the exit. This is because they had intended that I-40 would continue with I-85 to the interchange south of Greensboro, and the interchange where they split now, was intended as the exit for Business 40 & Business 85.

Either way, even with I-40 on the southern loop, I-85 still has the priority where I-73 / former I-40 exited off for the southwestern part of the loop.

On the eastern end of the I-40 / I-85 concurrency, I-40 also exits off with design favoring I-85 through traffic.

[GaryV]

I-69 loses twice in MI:
-- On the west side of Lansing, I-96 exit numbers apply  (I-69 exits the main line at both ends of the concurrency with I-96 exit numbers)
-- In Port Huron, I-94 exit numbers apply (I-69 is exit 271 from I-94)

[roadman65]

In Virginia I-64 loses twice with both I-81 and I-95.  It used to have no exit numbers in the state but the toll road in Richmond had sequential numbering from 1 to 17 at that time so only part of I-95 had numbers with a smaller part of I-64.

I-64 in Hampton once had a set counting up from the HRBT left over from when VA 168 was the sole route through Hampton.

[fillup420]

Your rule 4 probably has a higher priority in practice. For example, I-20/59 in Mississippi, the only logical choice is to use I-59 because its mileage is higher at the point they meet. If I-20's miles were used then I-59 would possibly have overlapping exit numbers. Similarly, I-75/85 is almost certainly because I-75's mileage at that point is higher than anything I-85 reaches on its own in Georgia. And not because of interchange design.

As for I-40/85, at least on the Greensboro end, the interchange is designed with I-40 as the exit. This is because they had intended that I-40 would continue with I-85 to the interchange south of Greensboro, and the interchange where they split now, was intended as the exit for Business 40 & Business 85.

Either way, even with I-40 on the southern loop, I-85 still has the priority where I-73 / former I-40 exited off for the southwestern part of the loop.

On the eastern end of the I-40 / I-85 concurrency, I-40 also exits off with design favoring I-85 through traffic.

speaking of Greensboro, the I-73/840 duplex has 73's exit numbers, but at the northern split, I-73 uses exit 107, which is it's own mileage...

[SkyPesos]

In Virginia I-64 loses twice with both I-81 and I-95.  It used to have no exit numbers in the state but the toll road in Richmond had sequential numbering from 1 to 17 at that time so only part of I-95 had numbers with a smaller part of I-64.

I-64 in Hampton once had a set counting up from the HRBT left over from when VA 168 was the sole route through Hampton.

Interestingly, I-64 loses in all of its concurrencies except with I-55 in MO, where I-55 exits off as exit 40B. I mentioned I-75 and I-77 in the op already, there’s also one with I-57 in IL that’s a rule 2 example in my guidelines, and there’s the 2 VA examples.

[cabiness42]

The 2009 MUTCD does not specify which interstate gets priority.

Where numbered routes overlap, continuity of interchange numbering shall be established for only one of the routes (see Figure 2E-21). If one of the routes is an Interstate and the other route is not an Interstate, the Interstate route shall maintain continuity of interchange numbering.

As for exceptions to the rules in your post, the first rule has a few:

1) I-465 gets priority over I-74 around Indianapolis
2) I-694 gets priority over I-94 north of Minneapolis.
3) I-294 gets priority over I-80 south of Chicago

I think it would be easier for a loop to take priority over a 2di that is concurrent for only part of the time. Would be confusing for a segment of the loop to suddenly have different exit numbers.

[US 89]

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.

[SkyPesos]

With I-35/I-70 (Alphabet loop) in Kansas City, anyone else count it as I-70 getting priority in the exit numbers despite that they’re both exit 2? For me, I do because I-70 enters the loop with the A suffix and goes in alphabetical order before leaving, while I-35 starts towards the end of the letters, and hops back to A when the concurrency with I-70 starts. The beginning and end points of the loop could’ve been marked at the SW corner to start I-35 at A, but it was chosen at the NW corner, to start I-70 at A. Think this is another example for my rule 4, since I-70 is longer in the state.

[SkyPesos]

I think it would be easier for a loop to take priority over a 2di that is concurrent for only part of the time. Would be confusing for a segment of the loop to suddenly have different exit numbers.

I guess it depends on the 3di. But in the case of I-694, I-640 and I-294, they’re unnecessary concurrencies with a 2di just to terminate at a specific route. Those can be truncated at where their individual segment ends.

[Caps81943]

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.

[SkyPesos]

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)

[Scott5114]

Thing is, if "lower number gets priority" were consistently followed, it would be common sense. It's much easier to figure out from the point of the concurrency which of two Interstates has a lower number. In some cases it can be hard to pick a "dominant route" or know which one is longer without looking it up.

[1]

Thing is, if "lower number gets priority" were consistently followed, it would be common sense. It's much easier to figure out from the point of the concurrency which of two Interstates has a lower number. In some cases it can be hard to pick a "dominant route" or know which one is longer without looking it up.

81 is clearly dominant over 64.

[Scott5114]

81 is clearly dominant over 64.

So what? That isn't true for every Interstate concurrency. What's dominant between I-35 and I-80? They both have one concurrency terminus where one is the through route and one is the TOTSO. The concurrency is L-shaped, so you can't go off of which highway's alignment it follows. Traffic count? What, Joe Traveler is supposed to look up AADT maps to figure out how exit numbers work now? What if you're going through at midnight and you're the only car around so the dominant flow is non-obvious? Or the flow is equal between the two?

Every interstate concurrency is, however, guaranteed to have one route with a lower number than the others, which will be evident to the motorist without the need to refer to any outside sources. At least unless TxDOT starts multiplexing suffixed Interstates, that is.

[hobsini2]

The one that always made me shake my head is the 90-94 concurrency in Wisconsin between Tomah and Madison. That is clearly an example of your #4 rule. At Tomah, 94 is the main line. At Madison, it's 90. But they use I-90's numbers.  The only thing I can think for the reasoning is because of the lower number idea. 90 is roughly 187 miles in Wis vs 94 being roughly 348 miles.

[KCRoadFan]

81 is clearly dominant over 64.

So what? That isn't true for every Interstate concurrency. What's dominant between I-35 and I-80? They both have one concurrency terminus where one is the through route and one is the TOTSO. The concurrency is L-shaped, so you can't go off of which highway's alignment it follows. Traffic count? What, Joe Traveler is supposed to look up AADT maps to figure out how exit numbers work now? What if you're going through at midnight and you're the only car around so the dominant flow is non-obvious? Or the flow is equal between the two?

Every interstate concurrency is, however, guaranteed to have one route with a lower number than the others, which will be evident to the motorist without the need to refer to any outside sources. At least unless TxDOT starts multiplexing suffixed Interstates, that is.

Between the two interchanges in Des Moines, the east-west distance is much greater than the north-south distance, so using I-80's mileage (as they do) makes sense there, in my opinion.

[SkyPesos]

81 is clearly dominant over 64.

So what? That isn't true for every Interstate concurrency. What's dominant between I-35 and I-80? They both have one concurrency terminus where one is the through route and one is the TOTSO. The concurrency is L-shaped, so you can't go off of which highway's alignment it follows. Traffic count? What, Joe Traveler is supposed to look up AADT maps to figure out how exit numbers work now? What if you're going through at midnight and you're the only car around so the dominant flow is non-obvious? Or the flow is equal between the two?

Every interstate concurrency is, however, guaranteed to have one route with a lower number than the others, which will be evident to the motorist without the need to refer to any outside sources. At least unless TxDOT starts multiplexing suffixed Interstates, that is.

The 2 examples for my rule 4 are L shaped concurrencies, where you can't tell which one is the dominant route. I thought the longer route in the state made sense, since that's what it seems like the most examples use. Besides I-57/I-70 and I-75/I-64, you mentioned I-80/I-35, which uses I-80's exit numbers. There's also I-55/I-72, which uses I-55's exit numbers

Another case is I-76/I-77 in Akron. I-76 exits off in a trumpet at one end of the concurrency, and I-77 exits off in a stack at the other end. The concurrency happens to use I-76's exit numbers even though I-77's mileage is larger at that point and is a longer route in the state than I-76. Maybe it's the lower number gets priority idea, but I think something else that gave I-76 the exit number priority is that the concurrency is entirely in the E-W direction, and I-76 is a E-W route.

[Revive 755]

The 2 examples for my rule 4 are L shaped concurrencies, where you can't tell which one is the dominant route. I thought the longer route in the state made sense, since that's what it seems like the most examples use. Besides I-57/I-70 and I-75/I-64, you mentioned I-80/I-35, which uses I-80's exit numbers. There's also I-55/I-72, which uses I-55's exit numbers

I-55 was around with those exit numbers many years before the I-72 designation was extended west to Quincy.

[TheGrassGuy]

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

[SkyPesos]

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.