My friend was telling me a few years ago, that people started saying 'thanks' when driving, by flashing their "emergency blinkers" on for a couple of seconds. I didn't know if this was widespread, or he was just trying to start a trend.
A few years later, I actually see a small number of drivers actually do this, ie. flash their "emergency blinkers" to show their appreciation.
Then again, there is still steady rate of drivers who simply do not ackowledge or show any appreciation towards your goodwill to them on the road, eg. giving way, ensuring the "keep clear" section is clear, etc.
I've never done a poll, but here goes…
Flashing hazards isn’t a thing where I grew up.
When you have completed overtaking you indicate right for a flash and then indicate left for flash to say thanks when a car slows to let you safely overtake.
Aside from that it’s either the wave or indicating depending on the situation to show thanks, because I’m not a profanity.