ftrplt,
Spotting an aircraft doing 500kts for collision avoidance is
not "eye out the window stuff", it's more commonly known as "near impossible".
Your best chance is knowing where that aircraft is likely to be
(via a functioning FIS), and avoiding that spot, or co-ordinating with them via radio, while you still have more than a millisecond to react.
And therein lies the problem.
