[zamber]

In Poland there's an unwritten custom of saying thanks by flashing emergency stop lights for two ticks. The most common use case is when someone lets you through when they don't have to.

On freeways the same is used by cars coming from the opposite direction if there's a radar ahead.

Still, in peak hours pretty much everyone is a dick, even usually chill Uber drivers.

Edit: it's also used as a "sorry" signal too.

Also, this is not local. I've seen it used all around the country.

[mantas]

Definitely not local. Pretty much a given in Lithuania too. If someone lets you in etc, it's just rude to not flash emergency lights. Even if somebody squeezed in like total dickhead during a rush hour

We use headlights to warn of friends waiting on the other side of the bush though.

[bencoder]

Also seen in London

[scaryclam]

And the rest of the UK :)

[themaninthedark]

Japan as well