It's not the first, I remember seeing decimal mileposts on OR 217 back in the mid 2000s on a trip to Oregon. Looks like they are still there, though I'm not sure if they were the enhanced mile markers like shown in the most recent streetview back then, so that may be newer.
Still good to know that I-84 has some now too though! Thanks for letting us know. Were the ones on I-84 regular mileposts, or the enhanced type with shields and cardinal direction like the 217 ones?
And I bet it's still the first Interstate that has them. I know for a fact that the whole of I-5 and I-205 didn't have them as of 2019 at the very least.
They were regular mileposts, without the shield. I'll try and get some pictures next time I get out that way.
It kind of makes sense to omit the shield on the 84, given the 30 duplexes for the majority of the route. Avoids milemarker 'overloading' (imagine it on the 84/30/395 triplex!).
Normally, the blue enhanced mile markers leave out all the multiplexed routes and only have a primary route on them, so just I-84 in this case (imagine 8 other routes with I-465). If it’s between two interstates, I’ve seen them either both on or just the one with exit number continuity on.
In fact, there shouldn't be more than one route on them, part of the point of having the route on there is to identify which route's mileage the signs refer to in a concurrency.
In Oregon, it'd be fairly moot, as the mileage doesn't belong to 84 or 30, (or 395 when joins the fray). It's 2, then 6.
30 effectively wouldn't be that affected, with the only noticeable discrepencies coming from the Mosier-The Dalles, Pendleton, La Grande, and Baker segements, for a total of roughly, what, 3-4? miles between I-5 and the respective crossings into Idaho in Ontario, as ORH 2's 0 is in Portland. And since 395 doesn't have its own undlying ORH number while it's on the 84, it doesn't care, as its next 0 is in Pendleton.
(This also puts 730's 0 at I-5, incidentally)
(ORH 6's technical 0 is at US 730, but like ORH 53 at US 26/OR 35, its posted 0 is in Portland)