Hard to believe that flipping the bird isn't on here :)

Likely they have not heard of the bird.

Not everyone has heard about the bird.

This is funny; however, it's not realistic.

When a driver yells or (hopefully inadvertently) runs a cyclist off a road, they are usually going much too fast for the cyclist to take any kind of action, positive or negative. At the rare chance they care enough to look in the rear view mirror, something innocuous like a hand wave might be perceived as an angry gesture.

The best answer is: do nothing. Cars are a huge danger for cyclists on the road, and in my experience it's best to just keep riding and move on.