It is a well known fact that the personal space, or bubble, of many people in the US is rather large. If people from other countries with smaller bubbles, e.g., Europeans, step closer than 3 feet towards a local, oftentimes the local person gets uncomfortable and takes a step back to reestablish the bubble.
It is very interesting, however, that the bubble can shrink quite dramatically without the affected person appearing to be too concerned about it. Point in case is when I went to New York City last weekend riding the subway from 110th street going downtown. At every stop it became more crowded. By the time we reached 34th street, people were densely packaged, like sardines in a can, and while people were making some faces, and there was some joking as to what might happen if the subway were to suddenly stop (nothing, because there was nowhere to fall down to), everybody seemed quite alright with the situation. Far more alright than I am sure any one person in that subway (including myself) would have been outside of it.
I am sure that as soon anyone stepped out, their personal bubble would expand instantaneously to three feet again. So why is it that in a subway (or, to a lesser extend, in an elevator) the bubble shrinks? Just some food for thought.