When should you signal before turning or changing lanes?
Well before the move, so others can see your intention in time
Give the correct signal well before you take the action and make sure other drivers can see it. Signalling is not enough on its own. You must also check that the way is clear before you move.
Why the other answers are wrong
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At the exact moment you start to turn
By then it is too late to help anyone. A signal is a warning about what you are about to do, so give it early enough that drivers behind you can slow down or make room.
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Only if there is another vehicle nearby
Signal every time, because you can never be sure the road is empty. A cyclist or pedestrian can sit in your blind spot, and the same rule covers slowing, lane changes and pulling out from a parked position.
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After you have started the manoeuvre
The signal comes before the move, it does not report a move already underway. Remember the order: signal, check that the way is clear, then move.
Quick way to remember
Early enough that others can react before you move.
Based on the official Ontario MTO Driver's Handbook.
Reading it once is not enough
Drill this question in a scored round and find out what else you are shaky on.
Practise Rules of the Road: The Basics