I always signal when changing lanes, even if I think I have the whole motorway to myself — I could be wrong. It’s habit, based on the assumption that any safety measure should be automatic so you do it correctly when under stress. (There’s a military maxim for this: “Train as you will fight, because you will fight as you trained.”)

When turning, I signal about 50 metres from the turn, or as soon as my turn is the next opportunity, whichever is closer.

When backing out of a parking space, I signal to show which way I intend to swing the arse end of the car.

I signal left (right in Britain and Ireland) when leaving a parallel parking spot to pull into traffic.