one of four most usual cause of fatal accidents in gliders
But only the 9th most common cause of accidents in GA. Unfortunately looking out of the window is only of limited usefulness though:
http://www.atsb.gov.au/media/4050593..._see_avoid.pdf
The exception being when you're in the circuit, and traffic is likely to be in a much more limited range of the sky.
We all know from experience that when we do see another aircraft out there in the cruise, often as not you catch sight of it at the last moment, then.. whoosh - it's gone before you could possibly have taken avoiding action. Or ATC asks you to 'report traffic in sight' and you can't find it even though you know more or less where to look.