There's something about driving that turns ordinary people into assholes. I've observed it in myself. Bus passengers are much more considerate, and I think that passengers in self-driving vehicles will behave like passengers, not like drivers. So if a pedestrian cuts them off they'll just shrug and keep playing their game or whatever they're doing.
I hope.
In a state known for Massholes it’s surprisingly unjerkish.
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.
We use headlights to warn of friends waiting on the other side of the bush though.
Game theory with no further interaction so no consequence to defecting other than the immediate.
The problem is that there is always somebody willing to be a jerk to save 2 minutes since nobody wanted to be stuck in traffic. The only defense against that is "Do unto before done unto" and that turns everybody into jerks.