The state rules are that you have to wear one inside any business/office/transit and outside if you can't maintain 6ft of distance. Looks to me like we have close to 100% compliance on indoor masking or while standing in line outside somewhere.
However, in Cambridge, you have to wear one in all public places, regardless of distance. The guidelines
even specifically mention biking and running. They also have this note:
"If you are exercising and need to remove your face covering to breathe adequately, make sure no one is near you before removing it. Be sure to keep the face covering readily available so you can put it back on quickly and properly if someone comes within 6 feet of you."
Sounds reasonable, but I don't think most people realize there's an acceptable situation for taking off your mask and are *really* uptight about it. Most of the scolding is just posts in neighborhood FB groups or NextDoor, but there are comments, gestures, and yelling occasionally outside, too.
It's not that much fun to take a mask on and off while running. It's very difficult on a bike. On local runs I've tried to go very early or late, and then most of the other people out are doing the mask on/off thing (though many just suck it up and run covered). There seems to be an unwritten rule amongst runners that if you are both unmasked as you approach then you both stay unmasked while passing (keeping distance, of course), so that helps. Still a lot more pleasant to head to a park elsewhere in the state for a trail run instead.
I do think part of why MA has been seeing such good progress in getting infections and hospitalizations down is because people are
so uptight about distance and masks, but I find it hard to believe that Cambridge's draconian rule including solo outside exercise is necessary.