OK, this one's nitpicky but here goes:
I suggest relabeling US45W_S -> US45W/51.
Reasoning: bleah, it's difficult to articulate, but... Consider the case of the east end of ME111, an example of a route ending during a multiplex, at some seemingly arbitrary place, not a numbered route. ME9_E would not be a helpful label here: WHERE during the ME9 `plex does ME111 end? Hence it gets named after the fixed point where it ends, MainSt.
I-69FutUni's case is a little different. It doesn't end at an arbitrary or unnumbered road, but at the US45W/51 split. Can't just call it US51_S though; that invites the same problems as calling it US45W_S. Thus I recommend US45W/51: pointing to the multiplex split gives the clearest reference to a fixed point, IMO. (Leaving aside the idea of it ending at some arbitrary point farther northeast on the 45W/51 plex. LOL!
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