Checked it out due to curiosity. Most of the freeway part of the Bradley connector (from I-91) is part of CT 20. Only about 0.65 mile of freeway isn't already in the HB. I think we would exclude the non-freeway airport service roads taking traffic from the freeway to the terminals.
Similarly, for the TF Green connector in RI (which I drove just a few days ago), probably only from I-95 to US 1.
Not necessarily reasons to not add either to the system, but the mileage involved might be less than you think.