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Concurrency Routes

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US 89:

--- Quote from: Amaury on October 04, 2022, 01:49:25 AM ---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.

--- End quote ---

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).

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.

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.

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.


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