Hampshire Cardiovascular Associates (Cooley Dickinson Building)
22 Atwood Dr, Northampton, MA
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Directions: Enter the building via the back; there's a ramp if needed. These are on the third floor (accessible via elevator or stairs). When the elevator opens to the waiting room, go left, then turn right, and there will be 2 on your right. Google says they close at 5.
Comments: I'm not sure where the other bathrooms are in the building, or if they're also gender-neutral. If asked, one could say that the bathrooms downstairs are full (assuming there are any), or that they're waiting for someone. (Or ask the receptionist directly where they are with/without an excuse, to prevent that possibility)