I checked the link to make sure I'd put up the right one...
Here's the bit that really stands out for me:
FAA Advisory Circular 90-48 C recommends scanning the entire
visual field outside the cockpit with eye movements of ten degrees
or less to en sure detection of conflicting traffic. The FAA
estimates that around one second is required at each fixation. So to
scan an area 180 degrees horizontal and thirty degrees vertical
could take fifty four fixations at one second each = 54 seconds.
Not only is this an impracticable task for most pilots, but the scene
would have changed before the pilot had finished the scan.
Harris (1979) presents even more pessimistic hypothetical calculations.
He estimates that under certain conditions, the search of an area 180
degrees by thirty degrees would require 2700
individual fixations and take around fifteen minutes
I'm not advocating keeping your eyes in the cockpit the entire time.
Just pointing out that doing so is hardly a guarantee of avoiding
collisions. The idiot pilot who didn't see you may well be keeping just
as good a lookout as you are!
In practical terms, the message I take home from this is that the
most important means you can use to avoid collisions is to rely
on other strategies such as the semicircular rule over MSA,
'random' cruising altitudes below and avoiding busy areas such
as gliding sites or nice-looking ridges. And in the circuit there's
probably more to be gained from looking in the directions you're
expecting aircraft to come from, rather than trying to scan for
random people passing through which would distract you from
the former...