A signal is a warning given in time to be useful. Given late it is an announcement, and an announcement is not a signal.
Three things must be signalled
A change of direction. A reduction in speed. A stop. Not just turns — slowing and stopping count, and your brake lights are what does that job.
Signal early enough that another driver can act on it. A signal that comes on as you begin the manoeuvre tells people what you are already doing, which is of no use to them.
The rest of this lesson
- Hand signals, and where they are banned
- Turning left
- Turning right