Two fingers or one thumb?

[Dayvo]

Take your pick from these:

http://en.wikipedia.org/wiki/Hand_gesture

My fave is the w@nker sign!

 

[alecstilleyedye]

i prefer raised pinky, wiggled, in a manner suggestive of the motorist concerned being short in some non car related area.

 

[Tynan]

in my world short bips on the horn are friendly and helpful, anything else is rude and agressive