Slow Waller's
adj: describing a group of 2 or more talkative pedestrians who obliviously create a slow moving, nearly impassable human wall across a sidewalk.

The phenomenon typically occurs at rush hour, near major metropolitan transportation hubs and frequently causes anyone who gets stuck behind one to miss their bus/train.
Jim: "Sorry I am late, I was a block away from the train station when I was trapped behind a 5 slow waller's.

Boss: "No problem, same thing happened to me last week"