I've got no problem with your complaint about them half-changing and blocking both lanes, it's the concept of nipping between them, knowing full-well their next move will be back in.
(UK) My instructor taught me to judge each situation. An indicator might cause more confusion. There is definitely no rule in the Highway Code that states you must always indicate. A system which relies on such an assumption is defective.
If you're intending to cross a lane marking you should signal. Under what circumstances would this not be true?
> Rule 103: Signals warn and inform other road users, including pedestrians (see ‘Signals to other road users), of your intended actions. You should always: use them to advise other road users before changing course or direction, stopping or moving off
https://www.gov.uk/guidance/the-highway-code/general-rules-t...
Isn't going into another lane, even partially, a change in course or direction?
However I do stand corrected as code does say that you must always indicate if you are changing course or direction.
"Signals warn and inform other road users, including pedestrians (see ‘Signals to other road users), of your intended actions. You should always
- give clear signals in plenty of time, having checked it is not misleading to signal at that time
- use them to advise other road users before changing course or direction, stopping or moving off
- cancel them after use
- make sure your signals will not confuse others. If, for instance, you want to stop after a side road, do not signal until you are passing the road. If you signal earlier it may give the impression that you intend to turn into the road. Your brake lights will warn traffic behind you that you are slowing down
- use an arm signal to emphasise or reinforce your signal if necessary. Remember that signalling does not give you priority."
I think this is because the driver was being a bit lazy and didn't bother turning on the signal. If you indicate intent, then the system won't think you're messing up.
But a lot of folks simply half-change lanes when passing cyclists and then go back.
As a motorcyclist the half-change is annoying since (a) you're blocking both lanes, and (b) if they did a full lane change I could zip past since I can fit quite comfortably next to the bicyclist.