I frequently find people flash at me while I'm looking the other way. Why do people flash at the back of my head?
When emerging, I've got a give way line, I wait, you go. (You meaning anyone on the main road, not you personally) Why rewrite the highway code for that occasion and expect others to fully understand. Even worse when they do it to one of my learners. Usual result is a stall. Learners don't need the extra pressure, really they don't.
When there are delays in others moving, it is I think due to a slow brain process. Someone flashes, there is a delay while the brain processes the information and computes a plan. Then a further delay to action that plan. Some of us will sit at a junction watching for opportunities and being ready for them. If you assume others are just as good as you this will always end in disappointment. They are all out to get you.
Similar scenario. You are driving along a main road. A car is waiting to join your road. The gap ahead of you is large enough for them to pull out. They don't move, then when you are close, out they come, gently, then wonder why you are on top of them looking upset. What is happening here?
They arrive at a junction to join a main road. Despite years of driving, they don't seem to know what to expect. They see a gap in the traffic that is large enough to emerge into. Their brain slowly processes this information, actions a plan to emerge and out they come. However, while the processing is happening, the gap is moving along at up to 60 mph, so has almost disappeared by the time they emerge. That bit is a surprise to them. So if the gap ahead is big enough, expect them to pop out late.
Some of us manage to look around and see a gap arriving and pop out at the start of it. Sadly, it is an impossible task to educate them all. I'm trying, really. So sometimes, on a main road it may be worthwhile closing down your safety margin ahead to discourage others from joining. Care is needed of course, to allow time and space in case the guy ahead stops to let the others out.
Driving late at night is a real pleasure. Most of the clowns are in bed.