Researchers at Harvard University tested fifteen people in such a scenario. The unsuspecting subjects of the experiment were walking down the street and a researcher dressed as a “passer-by” (who was in fact part of the research team) asked them for directions. As the subject was giving the directions to the passer-by, two workmen rudely barged between them carrying a door.
This highlights the fact that attention is primarily a conscious activity. The visual experience of change requires focused visual attention, which takes up a lot of processing capacity. In the “present moment” (this is the period of short-term memory span of 1-2 seconds) the conscious mind has a processing capacity of only 127 bits of information per second or about five separate items of thought (plus or minus two). That’s why we miss things at a conscious level. And that's why we need to pay more conscious attention to what's happening around us.