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[Amaury]

So, we know that in concurrencies, there's always a primary route, and the mileposts that are used are based on what the primary route is. The other route(s), however, is/are still counting, you just don't see it. So, if State Route begins a concurrency with US Route at State Route milepost 50 and US Route milepost 200, and the concurrency lasts for 30 miles, when the concurrency ends at US Route milepost 230, the next milepost after the split for State Route would be milepost 81, not milepost 51.

We can tell which route is the primary route based on which one comes first on the reassurance shields, whether it's on top or on the left. Here, for example, Interstate 82 is the primary route, since its reassurance shield is on top. Likewise here for US Route 97 and Washington State Route 20. This one is the same thing, but instead of top to bottom, it's left to right. And here, the primary route is Interstate 82, the secondary route is US Route 12, and the tertiary route is US Route 97. Likewise, here, the primary route is Interstate 90, the secondary route is US Route 2, and the tertiary route is US Route 395.

We also know that the importance goes interstates >> US highways >> state highways. So, my question is when the concurrency involves two or more of the same type of route, how is it determined which one is the primary route? Here, SR 20 and SR 21 are both local state highways, with SR 20 being the primary/more important route. In Montana, there's a concurrency with Interstate 15 and Interstate 90, and Interstate 15 is considered to be the primary/more important route.

[Quillz]

I’ve generally seen lower numbers get priority. But it’s by no means a consistent rule.

[amroad17]

I-64 has concurrencies with five odd north-south Interstate highways between St. Louis and the Hampton Roads area.  They are I-57, I-75, I-77, I-81, and I-95.  Each concurrency uses the odd number Interstate’s mileposts and exit numbers—more than likely because each of the odd number Interstates were completed before I-64 was.  However, in four of the five cases the higher numbered Interstate’s milepost and exit numbers were used.

Another example is I-20/I-59 in Mississippi.  I-59’s mileposts and exit numbers are used.

[Quillz]

We can tell which route is the primary route based on which one comes first on the reassurance shields, whether it's on top or on the left.

This isn't really reliable. In California, concurrencies technically do not exist at all. You have implied concurrencies, i.e. one route will exist but not signed. This isn't very consistent and more often than not, they do seem to be signed, but the order doesn't make sense. For example, during the brief CA-23/US-101 concurrency (more technically, CA-23 doesn't exist for a few miles while 101 bridges the gap), new signage in this area actually puts the 23 shield on top of the 101 shield. Not side to side. If anything, 101 shields should be on top. But they aren't. But then when you see CA-168/US-395, they are lined up side to side.

[Rothman]

I-64 has concurrencies with five odd north-south Interstate highways between St. Louis and the Hampton Roads area.  They are I-57, I-75, I-77, I-81, and I-95.  Each concurrency uses the odd number Interstate’s mileposts and exit numbers—more than likely because each of the odd number Interstates were completed before I-64 was.  However, in four of the five cases the higher numbered Interstate’s milepost and exit numbers were used.

Another example is I-20/I-59 in Mississippi.  I-59’s mileposts and exit numbers are used.

And yet, the MUTCD has guidelines regarding which mileposts are used.

[US 89]

So, we know that in concurrencies, there's always a primary route, and the mileposts that are used are based on what the primary route is. The other route(s), however, is/are still counting, you just don't see it. So, if State Route begins a concurrency with US Route at State Route milepost 50 and US Route milepost 200, and the concurrency lasts for 30 miles, when the concurrency ends at US Route milepost 230, the next milepost after the split for State Route would be milepost 81, not milepost 51.

This is not true in Utah, where state route mileage does not increase over concurrencies.

[Dirt Roads]

Along the Spine in North Carolina, I-40 uses I-85 mile markers because I-85 was there first.  I-74 uses the I-73 mile markers probably due to the north-south nature of the US-220 entire corridor.  But if you believe the signage plans, the northern end of I-77 might change over to use I-74 mile markers starting at the Virginia State Line.  (I'll believe it when I see it).

[1]

I see no reason to use the lower number. I would rather use the main route. For example, I-64 takes a jog when it overlaps I-81, while I-81's route is unaffected by the I-64 overlap.

There's not always a main route, though, e.g. I-75/85 in Atlanta.

[kphoger]

The Kansas Turnpike uses four different Interstate route numbers along its route:  I-35, I-335, I-470, I-70.

The 0-mile of I-35 is at the Oklahoma state line, and the mileposts remain consistent while it is concurrent with the Turnpike.

Where I-35 splits off from the Turnpike (at Emporia), its mileposts continue as expected.  Continuing on the Turnpike instead would be the theoretical 0-mile of I-335.  However, the exit numbers do not reset:  Emporia is Exit #127 for both I-35 and I-335.  There are no mileposts for I-335 below #127.  This means that the mileposts for the Turnpike remain consistent, and those for I-35 conveniently do as well.

But, at Topeka, it becomes abundantly clear that the Turnpike is the "primary" route when it comes to mileposts.

I-470's exits:
#1–#6 — western terminus to Topka Blvd exit
#177–#183 — concurrency with the Turnpike

I-70's exits:
#1–#366 — Colorado state line to K-4 exit
#183–#224 — concurrency with the Turnpike
#410–#423 — 110th St exit to Missouri state line

Interestingly, there are no duplicate exit numbers on I-70 in Kansas, but theoretically there could be.

[SkyPesos]

For concurrencies of two interstates, there isn’t a pattern and it’s up to each agency. But from my observations, a lot of cases have the longer route in the state getting the exit numbers.
Lots of exceptions to this like a historical reason that one route was here before the other (I-40/85), one of them clearly exits on both sides from the other (I-70/71, concurrency is in a E-W direction), etc.

With the above, I think the two I-90/94 concurrencies for which set of exit numbers to use are flipped; WI should’ve been with I-94’s, and IL with I-90’s.

[SkyPesos]

I-64 has concurrencies with five odd north-south Interstate highways between St. Louis and the Hampton Roads area.  They are I-57, I-75, I-77, I-81, and I-95.  Each concurrency uses the odd number Interstate’s mileposts and exit numbers—more than likely because each of the odd number Interstates were completed before I-64 was.  However, in four of the five cases the higher numbered Interstate’s milepost and exit numbers were used.

Another example is I-20/I-59 in Mississippi.  I-59’s mileposts and exit numbers are used.

Note that the short part of the I-55/64 concurrency in MO uses I-64’s exit numbers WB (no exits EB), one exception for I-64 that I see a lot of people miss.

[jmacswimmer]

I-64 has concurrencies with five odd north-south Interstate highways between St. Louis and the Hampton Roads area.  They are I-57, I-75, I-77, I-81, and I-95.  Each concurrency uses the odd number Interstate’s mileposts and exit numbers—more than likely because each of the odd number Interstates were completed before I-64 was.  However, in four of the five cases the higher numbered Interstate’s milepost and exit numbers were used.

Another example is I-20/I-59 in Mississippi.  I-59’s mileposts and exit numbers are used.

Just for the sake of nitpicking (because that's what we do here ), the I-64/77 concurrency is slightly more complicated in that since their mileposts ascend in opposite directions during the overlap, I-64 has the higher mileposts at the Beckley end while I-77 has the higher mileposts at the Charleston end. The "balance point" is roughly milepost 80 near the Chelyan toll plaza.

[US 89]

The I-15/90 concurrency in Montana is dumb. I-90 is set up as the through route at the interchanges at both ends, and 90 almost certainly has more traffic than 15. But mileposts and exit numbers use I-15 mileage, and the interstate spur into Butte is designated I-115.

[SEWIGuy]

In Wisconsin, the section of I-39 just north of I-90/94 has four exits that don't correspond to I-39's mileage at all - they are kind of numbered as if US-51 was duplexed with that section.

When US-51 begins its duplex with I-39 at the fifth exit, the exit numbers used are based on US-51's mileage.

I-39 is the first sign on all reassurance markers though.

[jp the roadgeek]

In CT, the more well known route usually gets the mileage (but not necessarily markers), with interstates trumping all.  Here’s a few significant ones:

US 5/CT 15: Uses CT 15 mileage
CT 8/25: Uses CT 8 enhanced mile markers (though they begin at the same point)
US 44/202 and US 202/CT 10:  Neither uses US 202 mileage
US 6/44: Uses US 6 mileage
US 7/202: Uses US 7 mileage
US 7/44: Uses US 44 mileage since it’s generally East-west)
CT 2/32: Uses CT 2 mileage

[Henry]

The Penna Turnpike is followed by four different Interstates: I-76, I-70, I-276 and I-95. Between New Stanton and Breezewood (the two endpoints for the I-70 concurrency), I-76's mileposts and exit numbers are used. They continue to increase after I-76 leaves to enter Philadelphia and I-276 begins bypassing to the north, and even when I-95 comes in to cross the Delaware and join the NJTP on the other side.

[NWI_Irish96]

In Indiana, both I-90 and I-94 take priority over I-80 as I-80 is entirely concurrent throughout the state.
I-65 takes priority over I-70 in Indy.
I-465 takes priority over I-74 (and I-69 when signed).

[JoePCool14]

For concurrencies of two interstates, there isn’t a pattern and it’s up to each agency. But from my observations, a lot of cases have the longer route in the state getting the exit numbers.
Lots of exceptions to this like a historical reason that one route was here before the other (I-40/85), one of them clearly exits on both sides from the other (I-70/71, concurrency is in a E-W direction), etc.

With the above, I think the two I-90/94 concurrencies for which set of exit numbers to use are flipped; WI should’ve been with I-94’s, and IL with I-90’s.

The 90/94 concurrency in Wisconsin is a toss-up to me. Neither route feels like the main route. In Illinois, I-90 "joins" I-94 on both ends. At least, that's what it feels like to me. I'm also biased considering whenever I head downtown it's on 94.

The one that I do find weird, but also understandable given how 294 should connect back to 94 and the tollway being involved, is the I-294/I-80 concurrency.

[SkyPesos]

The 90/94 concurrency in Wisconsin is a toss-up to me. Neither route feels like the main route. In Illinois, I-90 "joins" I-94 on both ends. At least, that's what it feels like to me. I'm also biased considering whenever I head downtown it's on 94.

I would still go with I-94's exit numbers in Wisconsin as it's mileage in the concurrency is 147-240, while I-90's is 45-138. Less repeated exit numbers when you're following I-90, while if you're following I-94 west to east and the concurrency uses I-90's exit numbers, you're repeating all mileage that you passed already. And I would imagine traffic to the Twin Cities on the western end to be more than whatever I-90 takes.

As for IL, with I-94 leaving the concurrencies on the right at both ends, it feels more to me that it's I-94 joining I-90. And I-94's mileage in IL gets nowhere close to the 85-100 range (what the concurrency exit numbers would be between if it used I-90's exit numbers.

This is also why I think the I-75/85 concurrency using I-75's exit numbers make a lot of sense, as I-85 doesn't get to the mid-200s exit numbers on its own in Georgia.

[SEWIGuy]

The 90/94 concurrency in Wisconsin is a toss-up to me. Neither route feels like the main route. In Illinois, I-90 "joins" I-94 on both ends. At least, that's what it feels like to me. I'm also biased considering whenever I head downtown it's on 94.

I would still go with I-94's exit numbers in Wisconsin as it's mileage in the concurrency is 147-240, while I-90's is 45-138. Less repeated exit numbers when you're following I-90, while if you're following I-94 west to east and the concurrency uses I-90's exit numbers, you're repeating all mileage that you passed already. And I would imagine traffic to the Twin Cities on the western end to be more than whatever I-90 takes.

While it's true that there is more traffic following I-94 than I-90 at the "northern end" of the concurrency near Tomah, the opposite is true at the southern end near Madison.

And the situation you describe with repeating exit numbers would occur for I-90 if you are heading westbound and using I-94's exit numbers.  It's a situation that can't be avoided no matter the choice.

[kirbykart]

The New York Thruway's mileposts are apparently more important than those of the Interstate routes that travel on it. Interestingly enough, the mileposts and exit numbers do reset on both I-90 and I-87, unlike the Kansas Turnpike situation mentioned upthread. And due to the Thruway's unorthodox directional routing, the mileposts start out okay at the southern end, but once you get to the east-west section what ends up happening is [pause to catch your breath] the mileposts are ascending backwards, with mile 120 or whatever in the Albany area, and mile 450 or whatever at Ripley at the PA line.

[Avalanchez71]

In Tennessee the US Highway concurrencies are based upon the secret concurrent state route that has through precedence.  So SR 1 would be the mileage used for the concurrency of US 41 & US 70S. For the short US 31/Business US 412 the through route is SR 7 until the split to SR 6. 

However, the split has just changed so that the through route for the concurrency is now SR 7 as US 412 splits of where SR 7 now splits.  SR 7 used to split off earlier and SR 6 took over, still with a precedence over SR 99.  The SR for US 412 is SR 99.  SR 99's mileage picks back up after the split (on both ends).

[vdeane]

The Penna Turnpike is followed by four different Interstates: I-76, I-70, I-276 and I-95. Between New Stanton and Breezewood (the two endpoints for the I-70 concurrency), I-76's mileposts and exit numbers are used. They continue to increase after I-76 leaves to enter Philadelphia and I-276 begins bypassing to the north, and even when I-95 comes in to cross the Delaware and join the NJTP on the other side.

FYI, former exit 358 is now exit 42.

[hbelkins]

Kentucky does not officially recognize concurrencies. The lower number of two routes on the same system always takes precedence. And mile markers for all routes except interstates and parkways reset at county lines.

The best instance I can think of where the lower-numbered route takes precedence is in Letcher County in the Isom community near the Knott County line. KY 15 is obviously the through route, and it has a signed concurrency with KY 160. But there is also a short concurrency with KY 7. That section of road is officially known as KY 7 in the official route logs and any mile markers posted would carry KY 7's mileage. Mileage stops and restarts at the intersections where the concurrencies begin and end. Good example is KY 11 and KY 15, which have a concurrency that runs most of the east-west length of Powell County. Both routes enter northbound from Wolfe County and intersect at Slade, adjacent to the Mountain Parkway's Exit 33. The concurrency through Stanton and Clay City carries KY 11's mileage. KY 15's mileage is "suspended," if you will, resumes where KY 11 north splits off. So you're about 20 miles into Powell County from Wolfe on KY 15 but the mile markers start counting up from around 4 or so.

The obvious exception to the "lower number on two same systems" rule is I-75 and I-64.

[gonealookin]

I’ve generally seen lower numbers get priority. But it’s by no means a consistent rule.

Nevada follows this rule consistently, along with the Interstate > US route rule.

US 6 is mileposted all the way across the state.

US 50 is mileposted except for its concurrency with I-580 in Carson City and with US 6 east of Ely.

US 93 is mileposted except for its concurrencies with interstates in Clark County, with US 6 and 50 east of Ely and with US 50 in Ely.

Et cetera.  US 95 is arguably the most significant US route in the state but it has several milepost gaps.