I have been meaning to make an entire web site about this idea for a long time. When driving, there are hand gestures to indicate frustration and anger, to acknowledge someone for letting you in, and not much else.

There really needs to be a universal gesture that indicates that "I made a mistake while driving, and I apologize for doing so and will do my best not to make it again. Thanks for noticing and not letting me cause an accident". I propose that this can be signaled by raising your elbow so the other person can see it. It is unique, and can be seen from far away. I am going to have to publicize it more if I hope for it to catch on. Tell your friends!