But a new LED panel aims to change that - letting drivers send a message via Bluetooth from their mobile phone to a grid of LED lights on the back.
At present, the grid shows pre-programmed messages - but its designer says he has plans to use it to 'Give feedback to drivers following me.
What manner of chaos this might unleash on the world's roads remains to be seen.
The pre-programmed messages include, 'Driving slow!' 'Keep distance!' and 'Turn left!'