Hi, the most obvious high traffic area is in the circuit, and joining or leaving the circuit. Try to get a good mental image of where everyone is by listening to the radio... Watch out for the double calls about simultaneous position reports... If someone calls 'G-aaaa on Final' just as you are about to give the same call, you can be pretty sure he is either above or below you. Best to give your position and height and your avoiding intentions.
If you fly the circuit at correct circuit height +- 30ft, then other planes will be silhouetted against the horizon. If they are 100ft lower they will be much more difficult to see against the ground.
Also be aware that we don't all fly the same circuit, nose to tail, like follow my leader. This makes the downwind leg a series of parallel tracks by all the different aircraft. Some will be just 300 yards from the runway, others will be over a mile away, depending upon aircraft type and pilot choice. If you fly downwind at 1000 yards out, expect other planes to be both to the left and to the right of you.
On a really busy day I have seen 5 in the circuit with 2 more waiting at the runway hold. It turned into a long wait, as a further plane joined downwind from a cross-country flight.