From Google’s Self-Driving Car monthly report:

The human act of honking may be (performance) art, but our self-driving cars aim to be polite, considerate, and only honk when it makes driving safer for everyone.

And:

We’ve even taught our vehicles to use different types of honks depending on the situation. If another vehicle is slowly reversing towards us, we might sound two short, quieter pips as a friendly heads up to let the driver know we’re behind. However, if there’s a situation that requires more urgency, we’ll use one loud sustained honk.

Our goal is to teach our cars to honk like a patient, seasoned driver. As we become more experienced honkers, we hope our cars will also be able to predict how other drivers respond to a beep in different situations.

This whole thing is fascinating to me. While most cars are capable of a single sound, usually tuned to F#, perhaps playable at different volumes, the next wave of cars will be capable of a much wider range of sounds.

For example, this HHR SS which is for sale can play engine noises if they sense a pedestrian is nearby, while electric cars are virtually silent.

And what use is a horn is the surrounding vehicles are also self-driving? Long term, I suspect that the horn is an endangered species.