Interesting. There's some signage southbound from MA28, though it disappears at the I-93 ramps. I found nothing northbound, including at the Sullivan Square interchange, which is poorly signed; nothing for MA99 either.
Perhaps most telling is,
despite the continuing southbound signage, milepost 0.0.
This reminds me of the semi-recent AARoads discussion,
Extension of MA 2A?As Jim noted in that thread,
I looked into this a bit more for CHM purposes. The 2012 MassDOT Road Inventory shapefiles still show 2A ending at Commonwealth.
Similarly, 2012 MassDOT Road Inventory shapefiles still show 38 ending at 28.
I may try to download newer shapefiles and see if anything's changed, but it's low priority.