Just a few further random comments from my practical experience, which includes a lot of hours flying gliders in the French Alps.
1. Our glider, like virtually all in the Alps, has dayglo strips over the wings. Makes the glider a little easier to catch with the Mark One
2. Gliders are often visible on radar but not that well. It is often heard that Brize (Oxon) can see gliders winch launching at Aston Down (Glos)
3. Gliders flying cross country do not have a predicable course - in the sense of a straight line. The pilots follow the best energy line, dictated by the clouds / thermals etc
4. From my recollection of nearly 40 years of gliding accident records and reports in the UK, whilst there have been a relatively high number of glider to glider midairs - often in the circuit - and by relative I mean relative to the rate of mid airs between aeroplanes, I can only recall four midairs in that time between a glider and an aeroplane. One about 20+ years ago in the Severn Valley area, another was a bit later near Thame. Both those two happened in clear air, not in cloud. The first was where the glider was hit from behind by the aeroplane. The third I recall was in the circuit at my home airfield, when sadly we lost a great friend. I cannot recall any midairs between gliders and aeroplanes in cloud. That is not to say it could not happen; just that the empirical risk appears to be low.
5. Flarm. In the UK it is catching on in the gliding world. I would hazzard a guess at c. 7 to 10% of the glider fleet are now equipped. But of course it relies on everyone else (or most) having it fitted also. In the French Alps it is far more prevalent and my experience so far with Flarm (three seasons) is that it is an extremely valuable additional aid to the Mark One. It is not a substitute. I have had a significant number of instances when Flarm has alerted me to other traffic well before I saw the traffic. It has the advantage of immediately indicating from which direction the conflicting traffic is coming and whether above, level or below one's own level. The big advantage of Flarm is it is instantaneous and does not need a third party (ATC, FIS etc) to intervene with all the critical time delays that involves. Mind you, in the Alps, flying close to the snow covered terrain often, it is very difficutly to see other gliders visually.
6. Unfortunately the regulators stick rigidly to the rule book when it comes to practical certification matters. Which is why we gliding folks wished we had not been subject to regulatory capture by EASA and all that goes with it. Flarm should be allowed to be fitted with the minimum of fuss from the certification boys. It ain't rocket science (to agree its fitment) even though the technology behind Flarm is, relatively.