Some basic rules of weapons training are to make the weapon an extension of yourself, and know where it is and what it’s doing at all times. This sort of training should be mandatory for anybody navigating New York City.
I have two modes of carrying my bag. For subway navigation, I hold it at my side, so it takes up the minimum amount of radial space, and so I can quickly move it in front of or behind me when the available corridors of motion start to constrict.
Once I’m at street level, if the foot traffic isn’t two dense, it goes over my shoulder, which is comfortable, but requires being aware that my shoulder is now extruding eight inches farther behind me and three or four inches farther to my right. This may seem a small amount, but navigation in the commercial districts of Manhattan during the day is often a matter of inches. A sudden twist of my torso could knock a fellow commuter to the ground, so I have to account for this extended space consumption when calculating safe turns and passages.
Most people fail to expand their kinetic awareness when encumbered with even their daily baggage. This is most often demonstrated by the standing spin, wherein a person with a bag of some kind is talking to someone, then suddenly needs to look in another direction, so they do a full turn and manage to block an entire sidewalk, often hitting a passerby with their bag in the process. The spinner simply doesn’t understand that they are consuming the space outside their body.
You are what carry.