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To Honk or Not to Honk, That is the Question

How a driver honks their horn can tell you a lot about what kind of driver they are.  Drivers who lay on their horn may be angry, frustrated, or impatient (1). People who honk excessively may be overly cautious or critical. Folks who don’t honk at all may miss the chance to send an important signal, such as a warning of caution, to a fellow driver. Honking is one way of communicating with each other on the roads we share, and we wanted to expand our AV’s repertoire to include the ability to honk. 

As we considered how we would teach our AV to honk, we thought about what kind of driver we wanted to be on the road. The answer: our AVs should be confident, careful, and responsible drivers who could connect us to the people, places, and experiences that we love – with as little drama as possible.

As a driver in a big city, we don’t want to add to the chaotic soundscape. But sometimes a friendly tap of the horn is all it takes to let a fellow driver know that the light has changed; that they should look up from their phones and be aware of their surroundings; or that they are about to create a hazard. Over time, good drivers have figured out ways to honk their horn to send a variety of messages. A quick tap of the horn may express a polite “excuse me,” whereas a long, drawn-out honk might urge a fellow driver to quickly change what they are doing.  

Where you honk also matters. Even a friendly beep may sound unfriendly if it’s honked near a hospital, or in a quiet neighborhood at two in the morning. On the other hand, depending on the level of ambient noise, there are some places where a honk may be the only way to get attention. The map below from the U.S. Bureau of Transportation Statistics shows the levels of noise pollution in the S.F. Bay area (2). The bright spots of purple are the noisiest, with more than 80 decibels of transportation noise on an average day. This is about as much noise as a garbage disposal makes (3).   

Source: Bureau of Transportation - National Transportation Noise Map

Why do we honk?

At first, we designed our autonomous vehicles to avoid honking because we didn’t want to add to existing noise pollution, and wanted to be mindful of the neighborhoods we drove in. Our AVs are patient and can wait out a situation instead of leaning on the horn.  

But as we collected driverless miles, we realized that many of our onroad incidents resulted from the inattention of other drivers. This included driving into or backing into our AV, often when our AV is stopped at a light or a stop sign; or patiently waiting behind a double-parked car.

Using the driverless road data we collected, we identified the use cases in which honking would help us avoid collisions. (For reference, 7 out of 36 driverless collisions in our first million miles were attributed to another driver backing into us.) We then designed our AV to use a timely honk in these types of situations to help others avoid a low speed collision with our AV.

When do we honk?

As we thought about how our AV should honk, our goal was to limit our honking to very specific scenarios to successfully prevent avoidable incidents but avoid adding unnecessary noise to our shared roads. 

To identify situations when honking would be appropriate, we first reviewed instances of onroad contact and identified instances where honking could have made a difference in the outcome. Then we took a closer look at each instance and characterized a number of factors that would determine how effective honking would be, including classification of the road user, distance of the road user from our AV, and the velocity of the road user as they approached our AV. We had to make sure that we weren’t honking too often in situations where the road user did not pose a risk of collision, but would honk soon enough that if they were approaching our AV, the human driver would have enough time to realize we were honking and react appropriately. 

Then we chose what type of honk would be assertive enough to sound like we were communicating with other drivers, but not so disruptive that it would create a ruckus in the neighborhood. While the Chevy Bolt platform supported a number of different horn sounds, we decided on the quick, double-chirp honk which was both friendly and attention-grabbing, and therefore the best fit for our use cases.

Finally, we tested the honking prediction model to determine whether we achieved the right balance between honking at preventable imminent collisions without causing unnecessary noise pollution and public annoyance. Once we were satisfied with our performance, we deployed our honking capabilities in August 2022 to our driverless fleet.

So what are the results? 

Since the release of the honking feature in August 2022, we have seen positive results. Post-release, the number of avoidable collisions that our AVs encountered that were preventable through honking decreased 5x. 

We also know that our AV doesn’t honk unnecessarily. We observed our honking performance for seven and a half months, between October 2022 to May 2023. Our current AV fleet consisting of over 200 AVs on average across San Francisco, Austin, and Phoenix honks an average of 6 times per day. 6 honks across more than 200 cars in three major cities! This gives us the confidence that our AVs mostly drive in peace, unless they see the need to honk.   

We know that our AVs can’t control how other cars drive, or prevent other drivers from being inattentive or speeding. But incremental, small steps like learning to honk are important building blocks to making our AVs, and our streets, that much safer.

So if you are on the roads of San Francisco, Austin, or Phoenix and hear one of our AVs chirp a “beep beep,” know that we are honking with intention and confidence (and politeness!) to prevent a fellow driver’s avoidable “oops” moment.