Firstly my sympathies to those related in todays accident.
Secondly, regarding the statements here about the density of traffic below/around controlled airspace. Has anyone got any figures to support the view that traffic density is getting any worse?
In my opinion things that are difficult to see in flight are:-
1 microlights
2 gliders
3 small helicopters
4 any aircraft on a fixed straight and level for long periods.
It is partly down to pilots to make themselves as visible as possible as well as looking and avoiding others. Gentle turns will display a bigger profile than someone sticking to a dead straight line on a GPS !!