I must admit, this is the only thing that scares me about GA.
I'm working for a PPL and my local area is very busy with traffic funneling into and out of the LLR between Manchester and Liverpool, and on more than one occasion I've had traffic prox of less than 500 meters. Not bad in itself, but both times I've only spotted them as they are looming in windscreen, certainly too late to really do anything meaningful.
I keep heads up, a constant scan, clear all my turns and listen out, but I still feel like we missed by chance. The rate at which you can close down another aircraft is surprising, one second it's a tiny speck, in another it's close enough to read the reg.
It's not always the case that you can do anything about it either - the last one was someone climbing below us on an almost converging track from behind. And those are just the ones that get noticed - how many go unnoticed?
Strobes, lights, whatever - there will always be one, once, that you miss, no matter how good your lookout is. If you think you'll never miss one, you're almost certainly wrong. It's like closing your eyes and running across the road, chances are you won't be hit, but would you take the chance?
I have no intention of going out for a few hours flying in the afternoon and never coming back. I'll almost certainly be buying parachute to go along with my PPL, because, well, **** happens.