Aussie Andy,
I've flown as aircrew (back in the 70's) and until 9/11 took FAM's whenever the opportunity presented itself, so I know how hard it is to see other aircraft. It still surprises me that the majority of all mid airs occur in VFR conditions, often in clear and unlimited visibility.
As others have said NDB's and VOR's act as magnets and traffic over or near these fixes can be quite heavy and pilots need to be extremely watchful for the other guy.
As for transition altitude if you are refering to when Flight Levels begin and pilots put 29.92 in their altimeter, this doesn't occur until FL180 in the US.
Mike