When will drivers learn??

[silverfoxcc]

On the A316 last night heading south, in the 50 limit just before the M3 starts. My lad is doing the limit in the inside lane, with a car over the '2 sec gap' in front. approaching the on slip and this idiot just anchors up full stop, like he hit a brick wall. We thought there was a problem traffic wise.
Oh No mr inconfkingsiderate, had in his infinite addled brain thought process decide to let the on traffic onto the A316, No thinking of moving out, if he wanted to, and he could have done no problem, as we went past there were several cars on the slip all nose to tail wondering what has happened
Why do drivers
1) think that oncoming traffic has the right of way, just because the rh indicator is on?
2) and/or either slow down/stop as this twonk did, or worse still change lanes, (and i bet without looking before changing.)

[P Nuts]

i think drivers in situations like that get nervous and panic when they see indicators etc and think they're going to do the right thing and be helping fellow drivers.