From here, where the author notes:
When the light turned green, the car ahead of me just sat there. I gave it a good five seconds (i.e., 0.5 seconds) and then blasted my horn. “What did you do that for?” asked my wife. “The light is still red.” So it was. I hadn’t noticed the righthand turn light. So, how do I apologize to the driver ahead of me? We lack a gesture by which we beg forgiveness. I propose the following:
When I do something dumb like that (and I frequently do), I gesture my embarrassment by wincing, smacking myself on the side of the head, and shrugging my shoulders. The other person either recognizes it as an apology or takes it as a sign of insanity. Either way, I feel like I’m covered.