You're making quick, purposeful strides on your way to an important meeting or assignation or whatever, when you have to stop short because of a slow-moving herd of tourist right in front of you. You get ready to zip around them, when blam: You have to stop again because another lumbering herd of tourist is coming at you head on from the other direction. You're trapped: You have to wait for the two herds to slowly shuffle past (and hope they don't get helplessly entangled).
It's especially acute on Boylston and Newbury streets, but a frustrated citizen complains about the tourist resting grounds on Huntington Avenue by the Pru:
The Duck Tours departing from Huntington Ave are no doubt extremely popular but there needs to be some effort to keep the sidewalks clear by seating those waiting. It's like trying to walk through a packed bar sometimes.