Joanna Carr's "busy cities" are certainly where this problem is most noticeable, but I've experienced versions of it even on hiking trails in the wilderness:
Joanna Carr, of London, writes, "I often wish I had a word (to mutter under my breath) for people with absolutely no horse sense when using public transport or in crowds. You know, the ones who get off the top of the escalator and stop dead, people who swerve into your path, people who walk four abreast. Surely a good descriptive noun is long overdue in busy cities everywhere?"
Suggestions in the form of comments are more than welcome. Or e-mail them to me.