There's only so much you can convey with gesticulation, flashing lights, tooting your horn and driving aggressively.
But revolution is only around the corner.
An Australian firm have come with a device (pictures at top of post) - they call it the Driv-e-mocion - that can send a variety of predefined messages (ie, it's not user programmable) via a wireless remote to the sign stuck to your rear windscreen.
It's not perfect - doesn't seem to cover your bulb's blown, baby on board or maniac - but it's progress.
via Shiny Shiny