The midair collision risk is always there as long as two aircraft are airborne in the same bit of airspace.
When I trained at Hamble in 1969 there was a fatal collision between 2 solo students - one was established in the circuit (right hand) and just turning crosswind - the joiner was a little bit wide and sadly they met. One of the recommendations of the report was that right hand circuits at a training airfield were not suitable when a/c with side by side seating were being operated, particularly solo. At the time there was full atc but it was a nice day and they often used "negative" r/t - ie just make the calls and atc would keep an eye out.
I think it's almost impossible to completely eliminate this risk but the answer lies in full and comprehensive training on all aspects of lookout and how to manage the flight (as englishal comments - intelligent use of r/t etc) - if you can't see the other a/c then ask!
Going back to the Hamble days there were often circa 12 in the circuit and probably double that over the IOW doing GH - a training situation which made you very aware of the importance of lookout and clearing airspace during climb/descent etc.
During the days when I did lots of basic instruction I observed that the lookout prior to commencing descent was often neglected - probably because instructors were not demonstrating it and/or insisting on it. Another one is those who descend on the dead side in a straight line - better to be turning so that you are clearing the airspace below and you have a better view of the runway and traffic which you might meet later on when crosswind so that you can adjust so that you don't do so.