Should/must - makes no difference - the use of a signal in no way alters the rules of priority .
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And I wasn't suggesting that at all.
The indicator only provides information.
If , as you come down the slip road , you are ahead of traffic on the carriageway , then they will clearly be able to see you and , since you cannot go anywhere other than onto the carriageway , the signal adds nothing to your presence , position and speed .
That's making an assumption about how other drivers are reading the situation.
Whilst I am sure this is not your intention , sometimes a trafficator signal given by a vehicle joining a motorway can be seen as aggressive by some drivers who will then 'close the gap' and this can be counter productive .
Again, that's making an assumption about how a driver might react to an indicator.
Motorway slips are not all the same. You can't assume that drivers on the motorway are paying attention. A flashing light indicating what you are doing does not detract from the situation and even though your intention should be obvious.
I'm much more worried about a driver daydreaming and trying giving them warning of my intent than the act of indicatign possibly being treated as aggressive.