When you flash your lights to let someone out of a side road and they sit there for 5 seconds looking at you. Flash lights again, still sit there staring at you. FFS.
While extending courtesy and inviting someone out of a side road onto a main road is all very well when the main road is at a virtual standstill with queuing traffic , such that the person in the side road might otherwise wait an eternity to get out ; it is one of my pet hates when well-meaning chumps stop without warning on a busy and free flowing main road to invite people out of side roads , possibly causing a concertina effect behind and even a crash due to following traffic not expecting the one in front to stop ( this is one of the few occasions where the driver in front can be held responsible for a rear end collision , because they did not have a valid reason to stop , did so without warning , and with a little forethought might have foreseen that following traffic might not have been expecting them to stop ) .
It is different , of course , particularly with larger vehicles , if a vehicle waiting in a narrow side road prevents or makes it more difficult for you to turn off into that road , and letting them go first makes it easier for you to make the turn - but then you would have been signalling your intention to turn off anyway so following traffic would have expected you to slow down anyway .
The people I object to are the ones who ( for example on my local village bypass ) just stop suddenly to let a lorry out of a side road , risking a rear end smash , then carrying on , when the lorry would have got out anyway a few seconds later when the half dozen vehicles on the main road had passed .
'Flashers' should also remember that , in inviting someone out of a side road in front of you , you assume a degree of responsibility for ensuring it is safe for them to emerge - remember you may be blocking their view - and coukd be held in part responsible if they pull out on your invitation to be hit by an overtaking vehicle who thought you had 'just stopped' and might not have seen the side road or the vehicle emerging .
While I never invite people out onto free flowing roads , I might do so on near static congested ones , but only by a hand gesture and never a headlamp flash , and also after making very sure there are no cyclists or motorcyists filtering down either side .
More often , I will just leave a gap , without signalling any invitation , and leave it for the driver in the side road to make their own decision to emerge or not .
Oh , and if I am in the side road , waiting to emerge onto a free flowing main road , and someone takes it upon themself to change the established rules of priority , I will quite often decline to come out in front of them , either by making eye contact and shaking my head or by a hand gesture indicating that I am waiting for them to pass first .