Signage for incomplete interchanges

[PHLBOS]

you are correct about the one-time Exit 17 on the Expressway.  It was the old connection to Frontage Road (NB only?) back when Mass. Ave Exit 18 was a left hand exit.

Correct, it was a northbound-only exit.

As part of the Big Dig re-configuration, the current Exit 18 now serves both Mass. Ave and Frontage Road, as well as the commercial vehicles only South Boston Haul Road.

In the southbound direction, the new Exit 18 also serves Andrew Square, which was Exit 16 (note: the Andrew Square exit was always Exit 16 since the Expressway was constructed).

Needless to say, the change-over of I-93 from the old Central Artery to the O'Neill Tunnel also meant an increase of incomplete/partial interchanges (including Exit 24- MA 1A North, the Callahan Tunnel) and even some ghost/missing sequential exit numbers along the corrdidor.  Such a change bolsters another reason to switch over to mile-marker-based exit numbering.