If you plan to slow down, you can warn drivers behind you by:
Slamming on the brakes.
Tapping the brakes a few times and before braking.
Turning on one set of turn signals.
Putting your hand out the driver's side window and waving.
Before slowing down, it is a good idea to flash your brake lights by lightly tapping your brake pedal a few times. This will warn drivers behind you of your plans to slow down.