A still-obscure gesture in our area is to use the index finger and thumb to form a circle while wiggling the other fingers in an upright position, which means "Flaming asshole!" I have used that for my own satisfaction when I don't want to arouse any road rage, but am really steamed at the behavior of another driver. Very satisfying!! What they don't know won't hurt me, but make them feel confused and annoyed, perhaps...

Mimsey