Thanks Charlie!
Before the new signal, the weave from where the NB frontage road ends at Cross St to the left turn lanes at (the Northern) Medford St intersection must have been a complete mess; absolute sterling example of myopic 1950’s era highway design!
In that case, the original designers probably realized space limitations precluded NB Medford St traffic from entering NB McGrath until Cross St. In that case, displacing where NB Medford crossed under NB McGrath would allow an at grade NB McGrath to still operate conflict free. So why not save a few bucks on steel and make it easier to tie in Poplar St? (There are/were 101 other reasons to not do any of this, but the infinitely wise 1950’s highway engineers didn’t consider those).