Following on from the various 'collision threads' I have done some eyesight checking -- and am depressed.
At every opportunity when I have had another aircraft on collision course (normally following someone on a departure - so not a big threat), I find it very difficult to see that stationary dot more than about 1.5 NM away. Even at 1 NM, I need to 'know' where the target is to see it. On the other hand, if a target has relative motion, I can usually pick it out up to about 4-5 NM.
In every case traffic has been called to me, I can only see it after relative motion has started to be noticeable.
The conclusion, I can't see en-route traffic in time to do anything about a collision - If I can see it, I am going to miss it anyhow!
Obviously coming near the circuit is different as everyone is changing course all the time, so you have a good chance to spot most traffic and avoid turning into someone.