I do that kind of downward turn of the lips smile a lot, that whilst still not technically a smile seems to convey a vague if not ambivalent sense of approval. I don't smile an awful lot. When I do it's usually over some inconsequential thought or small random act of kindness, lovingness or generosity that I've experienced or seen in others.
Yeah, I recently almost hit someone at a 4way stop because they did that very same thing. It was my turn to go left, after the car in front of them. They took off right after the car in front of them took off, almost hit me and looked at me like I was a damn fool while they blew thier horn. Wait your turn fuckface!
There are two -- (how in the world would a non english speaking person pronounce two?) ---places where you never hear a car horn and people always let others into traffic from side roads or driveways: Oregon and Hawaii. Makes one smile at the unselfish actions.
I don't remember how polite they were as drivers, but they sure treated me well all across Canada when I went from Vancouver Island to Montreal in the summer of 1970. Great times. And I never got stuck in Wawa, as I heard many hitchhikers say.