The figure is based on flying under a radar information service and seeing how many of the reported contacts are "level unknown".
The % clearly varies with the altitude. Probably below 2000ft the % is at least 90%. If you encountered a lot of microlights it would be higher still. With gliders it would be close to 100%.
Above say 5000ft, nearly everybody is transponding but IMHO that reflects the different sort of pilots, different planes and probably also flight rules (not many VFR-only people will be flying at 5k in the UK) that fly higher up.
I would buy TCAS tomorrow if transponders were mandatory for VFR. But they are not and - without wishing to start another Mode S thread - I can't see that they will become mandatory for microlights and similar. However, few people flying those types (that I think will be exempted) fly above say 2000ft and flying "high" is thus a very good defence for traffic
enroute. So the case for TCAS OCAS in the UK isn't that good IMHO, for for the installed cost of £10k-15k.
The risk with terminal (circuit) traffic remains and I suppose the usefulness of TCAS will depend on the traffic mix you get at the airfield you are going to. If it is microlight-heavy then TCAS will be of little use. You just have to keep the best possible lookout and hope for the best, for the few minutes that you are in the circuit.
Robin - the Mk1 eyeball so beloved of aviation traditionalists is actually pretty useless, but let's not ruin such a fondly held and propagated legend that has seen off Germany in two world wars
A plane on a
true collission course will be a completely stationary point, until the impact. I think this is a problem which just does not have a comprehensive solution.