[PuffinBlue]

They are, at least on my car and many other here in Europe. Brake past a certain threshold of deceleration, and the brakes lights flash rapidly, indicating to motorists behind that rapid deceleration is occurring. I’m not sure what the threshold is.

[bdsa]

In my experience the threshold is high enough that it only helps you distinguish between "incredibly sudden but non-emergency stop" and "full-on emergency stop"

[gnicholas]

What kind of vehicles do you see this on? I have only rarely seen this, perhaps because it's not common in the US, or perhaps because emergency braking isn't all that common. But if there's going to be a different kind of light behavior, I'd think they'd want to have it triggered commonly enough that people would know what it means. Otherwise they might be confused the first time they encounter it.