If you turn your turn signal on while there’s a car in your blindspot that’s just bad driving. The other driver has no way of knowing if you’ve seen them or you’re about to swing out on them.
Disagree. The purpose of the indicator is to signal your intention to others. Indicate first, then shoulder check. If you then discover a car in your blind spot, abort the manoeuvre if necessary or slow down to let them pass you first.
I do agree it's bad form to indicate when you know there is a car in your blind spot already.
Discovering a car in your blind spot should be a relatively rare occurrence if you are driving attentively.
You indictor is an indication of your intensions so that the other drivers are not surprised when they see you car move from the expected path. If you are only a road with light traffic then yes, first adjust your speed so cars are not in your blind spot and then indicate.
But, if you are in tight traffic then a gap will unlikely open up. In this case you use your indicator to signal your intention, and wait until somebody chooses to let you in - making sure they have noticed you before moving in.
A car that beeps at you just because there is a car in your blind spot when you indicate is not suited to Australian road rules.
I do agree it's bad form to indicate when you know there is a car in your blind spot already.
Discovering a car in your blind spot should be a relatively rare occurrence if you are driving attentively.