Mind you, that this is not enabled in all countries. In the USA the rear fog lights turn on in addition to the brake lights under heavy braking, as flashing the lights weren't allowed.
They remain on until you no longer depress the brakes.
Unsure if they're allowed right now, or whether the ones you see are aftermarket mods.
For BMW you can change the coding (configuration) relatively easy.
There is also a difference in 3 flashes when you touch the brakes, constant flashing while pressing the brakes, or only flashing under having braking.
I can't imagine driving in stop&go traffic where all brake lights are constantly flashing.
In Europe, a lot of modern cars have a similar feature. When braking particular hard (eg: a sudden emergency stop from highway speeds) the brake lights will flash rapidly to warn drivers behind you. It’s a very good safety feature, IMO. But not legal in North America?
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