Only in these two states...

[US 89]

Arizona and New Mexico:

No 3DIs

Alaska

[OCGuy81]

CA and NY: only 2 states with 3-dis that start with a 9.

[hotdogPi]

CA and NY: only 2 states with 3-dis that start with a 9.

I-985 in GA

[froggie]

Washington and Oregon: centered exit tabs.

Until recently, Minnesota.

[OCGuy81]

CA and NY: only 2 states with 3-dis that start with a 9.

D'oh! Forgot about that one. 😂

I-985 in GA

[webny99]

ARGUMENT TIME

Which two states' panhandles actually deserve to be called panhandles?

Florida and Oklahoma. Is there even more than two options?

[ran4sh]

Washington and Oregon: centered exit tabs.

Centered exit tabs were the norm everywhere until someone got the idea to align it based on direction and got that guidance added to the MUTCD. Technically the right-aligned exit tab for a right exit is still only Guidance in the MUTCD, although in the proposed new MUTCD it is proposed to be upgraded to Standard.

[Caps81943]

California and Vermont: Green state route shields (if we're considering states only)

South Dakota says hi.

Texas and New Mexico: only states that use horizontal traffic signals consistently throughout the entire state (barring a few exceptions)

Wisconsin is iffy for this, 15 years ago I'd say Wisconsin was a member of the exclusive horizontal signals club, but it's moving to vertical now.

Though I'd stick Nebraska with TX and NM.

[KeithE4Phx]

Arizona and New Mexico:

No 3DIs

Alaska

West Virginia and Vermont

[ran4sh]

Tennessee & South Carolina: State route markers that are always the 3-digit width even when used for 2 or 1 digit routes.

(A related standard, using a 2-digit width even for 3+ digit routes, is done in plenty of other states)

[ran4sh]

Arizona and New Mexico:

No 3DIs

West Virginia and Vermont

West Virginia has a 3di off I-70

[cl94]

Arizona and New Mexico:

No 3DIs

West Virginia and Vermont

West Virginia has a 3di off I-70

And Vermont has I-189.

[SkyPesos]

Another one
OH and PA: only 2 states with at least 5 2di interstates with the same number first digit (6 I-7x for OH, 5 I-8x for PA)

[jakeroot]

Washington and Oregon: centered exit tabs.

I'm not aware of other states that center their exit tabs. WA uses full-width internal exit tabs*, whereas Oregon uses actual tabs mounted above the sign, but both centrally-mount the legend and, in the case of Oregon, the actual tab itself.

* exception being SW WA, which does things weirdly and likes to use actual tabs mounted on the right edge of a sign as is normal elsewhere.

Georgia also has centered full length attached exit tabs. Them and WA are the only states I know of that does this, so those 2 can be a pair for this thread.
In the past, Missouri also installed centered exit tabs, similar to Oregon's, but it's all on the sides now. There's still a couple examples left in Missouri though; here's one at I-270 and MO 340

The 1971 Los Angeles exit numbering experiment had centered tabs as well:
https://www.aaroads.com/forum/index.php?topic=3442.0 (thread I began on that topic in 2010)

Until recently, Minnesota.

I want to make it very plain that I recognize centered exit tabs as having been a thing in many places historically. But even in the cases highlighted above: MN, CA, GA, CT...none still use centered exit tabs or centered exit legends. Georgia seeming to be the most recent of those to shift away from centered exit tabs (which, apart from using Series D, were identical to WSDOT's design).

But I believe I'm still correct in saying WA and OR are unique in that they continue to use centered exit tabs; it wasn't something one or the other has done for a long time or has a lot of remaining examples of. It's literally as simple as "WA and OR use centered exit tabs...period".

froggie: could you be more specific when you say "until recently"? I'm seeing exit tabs in MN that are right/left justified on GSV going back to 2008.

[CNGL-Leudimin]

Arizona and New Mexico:

No 3DIs

Alaska

Alaska and New Mexico: Have no idea of what 3dis are (Seriously, they haven't planned one yet).

[Flint1979]

Another one
OH and PA: only 2 states with at least 5 2di interstates with the same number first digit (6 I-7x for OH, 5 I-8x for PA)

New York has five 2-di's starting with an 8 as well.

[jmacswimmer]

Michigan and North Carolina: only two states with diamond shaped highway markers?

Aren't they also the only states that sign their central business districts "DOWNTOWN" on their highway signs? Or does another state do that as well?

I recall seeing that along I-10 and I-45 in the Houston area

Colorado has some of those, too. On I-25, exit 142 is Bijou St/Downtown for Colorado Springs, and southbound exit 213 is 38th Ave/Park Ave/Downtown for Denver. Some express lane signs reference "Downtown"  further out, like here on US 36.

Downtown signage off of 110/Harbor Freeway northbound past I-10 in Los Angeles:
https://www.google.com/maps/@34.0402227,-118.2735916,3a,75y,39.3h,97.26t/data=!3m6!1e1!3m4!1skAda1l92hX2u-ASVNj1dug!2e0!7i16384!8i8192

Downtown signage in Maryland:
-Baltimore from I-95/I-395
-Cumberland from I-68
-Frederick from I-70

[froggie]

froggie: could you be more specific when you say "until recently"? I'm seeing exit tabs in MN that are right/left justified on GSV going back to 2008.

From what I can tell, MnDOT changed their guidance in 2015.  But as recently as 2 years ago (the last time I made it back), center tabs were still plentiful.

[formulanone]

Florida and Indiana: A strict grid numbering system

I think they're also the only two states which officially use "State Road" instead of State Highway or State Route. Could be wrong about this, but I can't find the specific thread where we discussed how DOTs officially named their state inventory.

Mississippi also has a grid that's about as strict as Florida's, except for two (10 and 20) which were re-numbered for Interstates.

CA and NY: only 2 states with 3-dis that start with a 9.

I-985 in GA

The hidden designation of US 90 Business in the New Orleans area is I-910.

[tdindy88]

Michigan and North Carolina: only two states with diamond shaped highway markers?

Aren't they also the only states that sign their central business districts "DOWNTOWN" on their highway signs? Or does another state do that as well?

I recall seeing that along I-10 and I-45 in the Houston area

Colorado has some of those, too. On I-25, exit 142 is Bijou St/Downtown for Colorado Springs, and southbound exit 213 is 38th Ave/Park Ave/Downtown for Denver. Some express lane signs reference "Downtown"  further out, like here on US 36.

Downtown signage off of 110/Harbor Freeway northbound past I-10 in Los Angeles:
https://www.google.com/maps/@34.0402227,-118.2735916,3a,75y,39.3h,97.26t/data=!3m6!1e1!3m4!1skAda1l92hX2u-ASVNj1dug!2e0!7i16384!8i8192

Downtown signage in Maryland:
-Baltimore from I-95/I-395
-Cumberland from I-68
-Frederick from I-70

What I meant to say was that both Michigan and North Carolina sign their downtowns by spelling out Downtown in All-Capital Letters.

Such as here in North Carolina: https://www.google.com/maps/@35.5551793,-82.6161122,3a,75y,94.5h,91.67t/data=!3m6!1e1!3m4!1sOnyMRYxs5DPnV7CR54x29Q!2e0!7i16384!8i8192

And in Michigan: https://www.google.com/maps/@42.9189415,-83.6775916,3a,75y,330.55h,86.33t/data=!3m6!1e1!3m4!1sxgaXjyFrv0A_MTajxqnLWQ!2e0!7i16384!8i8192

All those other examples spells the word Downtown in mixed case letters while these two both do them in all caps.

[roadman65]

GA and TN actually sign hidden state routes on US Highways.

[US 89]

Florida and Indiana: A strict grid numbering system

I think they're also the only two states which officially use "State Road" instead of State Highway or State Route. Could be wrong about this, but I can't find the specific thread where we discussed how DOTs officially named their state inventory.

I think New Mexico is also a "state road" state.

GA and TN actually sign hidden state routes on US Highways.

TN does not. GA does very reliably, to the point that in multiple cases the state route number is the local name for the road rather than the US number. GA 365 instead of US 23, GA 400 instead of US 19, etc...

[jmacswimmer]

CA and NY: only 2 states with 3-dis that start with a 9.

I-985 in GA

The hidden designation of US 90 Business in the New Orleans area is I-910.

And as discussed in the "District of Columbia" thread, DC just passed up a golden opportunity to introduce I-995! 

[Henry]

Arizona and New Mexico:

No 3DIs

Alaska

Alaska and New Mexico: Have no idea of what 3dis are (Seriously, they haven't planned one yet).

Are we going to ignore the once-proposed I-410 and I-510 in Phoenix, or I-710 in Tucson? While AZ currently doesn't have any 3dis, it certainly has planned some. NM, on the other hand, has never had any such proposals that I know of.

[kphoger]

I don't think anyone ever said Arizona hadn't planned a 3di.