Don't kid yourself that midairs only occur in a busy circuit.
I was at Hamble in 1969 when two students collided and were killed. I think there were about two in the circuit with one joining.
One of the recommendations of that report was that right hand circuits should not be flown at training airfields with aircraft with side by side seating with the commander sitting in the left hand seat (solo).
Radar would be pretty useless in this situation but I recall that in the States quite a few years ago the biggest likelihood of collision was within 10 miles of an airport with mixed IFR/VFR traffic.
One answer is to limit the numbers in the circuit and also have plenty of relief landing grounds where early solo students can fly students without the congestion problems.