When should you flash your brake lights to warn drivers behind you?

Tapping the brake pedal lightly to flash the brake lights is a deliberate warning to following drivers when you are about to slow significantly, when there is a traffic jam ahead they may not see, or when an obstacle requires you to brake harder than usual. The early warning gives the driver behind extra seconds to react. This is especially important in commercial vehicles because following drivers often misjudge how quickly a heavy vehicle can slow and tailgate accordingly. Flashing brake lights when stopped or barely moving in traffic communicates to drivers approaching from behind that they need to slow. Continuous brake light operation when not braking is illegal and confusing.