In reply to FesteringSore:
> Conversely, when I fly any where I often look out of the window and, with a few exceptions, it looks as though the skies are totally devoid of aircraft except the one you are in. Just occasionally I have seen a distant contrail or an aircraft passing by at a(thankfully safe) distance.
Thats not my experience - I seem to see loads of other planes when I am flying. Particularly when queuing to go into big airports, once you are on the stack for Heathrow for example you can see all the other aircraft going round the racetrack descending slowly until you go in to land. But on flights to the states I've seen a lot of other planes, some overtaking.
The ones you don't tend to see as well are the ones going in the opposite direction but you can spot them with patience, even at significant height differences they are really shifting. Back before going to see the pilot was forbidden I was on the flight deck of a 777 when a small jet came the other way just 1000 ft above, that was eye-opening.