One of the problems for passengers, or indeed for any of us, I believe is that the standard seperation of 1000ft vertically seems like it should be a huge margin. The scale of the number and the fact that little else in our lives is measured in feet anymore gives us little perception of what 1000ft actually is like. The one place we do come across it frequently, if you would like to use your imaginations, is at a motorway junction. The countdown markers prior to the junction are 300/200/100 metres. 300 metres is approximately 1000ft. So now imagine 1 Boeing 747 at the 300 metre board and a second one at the actual turn off and you will see that it is not as large a seperation as we may have thought.