What should you do to alert drivers behind you if you are going to stop or slow down unexpectedly?

To alert drivers behind you that you are going to stop or slow down unexpectedly, tapping your brake pedal three or four times quickly is the most effective method. This action provides a clear visual indication to drivers who may not be expecting your change in speed. The rapid flashing of your brake lights can capture their attention and signal that you are slowing down, thereby helping to prevent potential rear-end collisions.

Using your turn signal is appropriate for indicating a turn or lane change, not for showing a sudden stop. Sounding your horn can be used in urgent situations, but it may not be effective for conveying a change in speed to those behind you. Flashing your headlights is generally used to communicate with oncoming traffic rather than to signal braking, making it less useful in this context. Therefore, the practice of tapping the brake pedal is the most reliable way to inform those following of an unexpected stop or slowdown.