It's good to use your turn signal.
Erika is driving on a busy road.
Erika needs to make a turn.
Erika signals to make a turn, then begins slowing down.
Erika doesn't signal and slows down to make a turn.
The person behind Erika sees her turn signal and switches lanes.
The person behind Erika didn't realize she had slowed down and rear-ends her car.