Does anyone know how the custom began of flashing one's headlights to inform other motorists about police cars lying in wait for speeders? This is something that predates the internet, cell phones, or other electronic mediums for quickly spreading a code or signal, so I surmise that it was spread by word of mouth or something equally non-technological.

What does this have to do with this thread? I suspect that similar signals are used in Iraq and Afghanistan that coalition forces are unaware of, but that are common knowledge to the locals.