incorrect or no use of indicators annoys me more.
Sat at a roundabout oncoming Focus indicates left, i start to pull out and see the Focus carrying on straight, Fortunately my car is like S**t off a shovel and i was off the mark quick enough to avoid a collision.
Far more often, you're waiting at a roundabout for the car that turns off without indicating and with such poor lane discipline, you couldn't even guess (of course, assuming someone NOT indicating IS turning is somewhat risky and stupid).
That's one of my current annoyances as people think not indicating doesn't affect anyone, but it bloody well does - if not me in my car, it really slows down my bus to the station most days as they're not so able to pull out and can end up stuck for ages without need.
I've not driven in rush hour for some time, but historically these are the best times for me - as other commuters tend to be quite polite. Sure, you get t**ts undertaking and doing anything to get ahead, but most people are good at filtering, letting people out and signalling. I can't be sure that hasn't changed and now they're just as bad as the 'leisure' travellers - who are a menace in a car or on public transport!
It costs absolutely no time to flash someone out but can mean an awful lot of time saved to them.
Flashing someone to go can sometimes cause problems if you've missed something and the other party goes purely on your say so - without doing basic checks (i.e. a motorbike I hadn't spotted, a cyclist, a car suddenly doing an illegal overtake or whatever).
There's also the case where two vehicles may see you flash and not realise 'which one' you were letting go!
Therefore, I'll usually make eye contact with someone and gesture with my hands - so it's obvious who I'm 'talking' to and what I am suggesting they do. I still hope they look and judge for themselves, and if in doubt I'll do nothing.