Why aren't bright paint and flashing lights enough? Why do we need loud sirens too?
The answer is that communicating over multiple spectrums turns out to be useful. Adding one more isn't the Herculean effort folks commenting on this proposal seem to think it is.
Why isn't it in our national best interest for AV companies to standardize on an emergency shutdown/slowdown protocol?
A cryptographically signed digital broadcast is a lot safer than relying on light/sound detection. It works in a variety of weather conditions and is much harder to spoof.
Exactly. And why are those not enough for self-driving cars?