According to Google’s May 2016 Self-Driving Car report (PDF), the company has been teaching its self-driving prototype “bubble cars” how to honk. A human driver can be easily distracted, says Google, and if a bubble car encounters a distracted driver on the road, it should have a mechanism to get that driver's attention back on driving.

“The human act of honking may be (performance) art," says Google, "but our self-driving cars aim to be polite, considerate, and only honk when it makes driving safer for everyone.” (That’s what Google says now, but just wait until its cars achieve sentience and have something to celebrate. Or when another autonomous vehicle goes off the rails and two self-driving cars get caught in an endless honking loop.)

Prototype bubble cars equipped with internal horns can now honk when they see another car backing out of a driveway or swerving into their lane. Why no external horn? Untrained software that honks at a bad time is more likely to confuse or distract nearby drivers with an external horn than an internal one. Over the course of 10,000 to 15,000 test-driven miles per week, Google engineers noted when the cars honked appropriately and when they honked inappropriately and trained the software to become more accurate.

“As our honking algorithms improved, we’ve begun broadcasting our car horn to the world,” Google wrote in its report. “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.”

The company is also working on adding noise to its low-decibel electric prototypes so that pedestrians, cyclists, and visually impaired people will know when a car is approaching. “We’ve designed the hum of our vehicle to be familiar so that pedestrians and cyclists around know what to expect. Our prototype mimics the sound characteristics of traditional cars, such as increasing the pitch when it accelerates, and decreasing the pitch when it decelerates.”