I followed a link to a Google report about the safety record of self-driving cars. It relates (brags about) one incident where one cyclist swerved from the left turn lane into the Google car’s lane, while a second cyclist traveling against traffic on the cross street turned left into a collision course with the Google car … and all this was at night. The Google car was able to identify the danger and stop in time to prevent an accident. Good for it, but they didn’t mention a honk. Now, sure, human drivers frequently use the horn to say, “Hey jerk, you drive like my grandpa, and he’s been dead for 20 years!”, but it is intended to be part of the car’s safety equipment. Saying “Hey, you drunk and/or insane cyclists, you’re putting yourselves (and my paint job) in danger. Pull up. Pull up!”, would be entirely appropriate in this situation, and might be necessary to prevent an accident the car couldn’t avoid on its own.
Google could equip these self-driving cars with oversized silhouettes of (human) hands, showing a middle finger. This would be less annoying then honking.
When a horn is used, it’s supposed to be annoying. That’s its purpose: To be noticed by other drivers and to let them know that something is wrong.
Or, in most cases of horn usage, to let them know that they are assholes.
Well, yes, that’s the something wrong that they’re alerting of.
I was traveling in a Taxi when the driver used his warning device. He tapped in once as he was coming down the hill, to alert a driver who was parking just ahead of us. My driver used his warning device BEFORE the other driver backed out in front of him, in ANTICIPATION that a driver doing parallel parking might saw back-and-forth a few times while settling into a parking place.
I was surprised. It was memorable. I’ve never before or since seen a driver use a horn like that.
No, you only want that to be displayed some of the time.
So the proper engineering from Google would be a hand where each finger can be individually controlled. Probably could be done by rewiring the standard traffic light pedestrian DONT WALK signal, thus reducing the cost.
Come to think of it, one “warning” instance I have seen, when travelling on a bus in winding mountain roads in Italy: The bus driver honked at every sharp turn, the idea being that the road was too narrow for two vehicles to pass without really paying attention, and the sight distance ahead was to short to stop if you were travelling at any reasonable speed, so the horn was to let any oncoming driver know that there was another vehicle just around the bend they’d need to avoid even though they can’t see it yet.
So Google cars would need to have a horn for that purpose, at least.
Drivers in the Philippines use their horns thusly, at the slightest excuse.
There is a curve near where I life that has a sign stating honk horn. I think the Googlemobile would have to comply with such local laws, but perhaps the human passenger would be the one expected to do this.
My experience is that behavior happens in most of Asia.
I am in China now, and many of the drivers here seem to be honking their horns almost constantly.
Sorry to continue the hijack, but India is possibly the worst for this, as there is a convention that drivers do not look in their mirrors, so any time you even come close to another vehicle you should honk your horn (many vehicles even have a sign on the back saying “Please honk horn” or whatever, and I don’t think it’s a euphemism).
I sure hope I’m being whooshed.
'Cause it ain’t such a memorable event…really.
It’s a handy technique…And not just for people backing out of parking spaces. It works when you see a blind person on the side of the road. And when you see a bicycle rider on the shoulder. Or a group of children standing too close to the curb.
But it doesn’t work with cats… Never honk at cats. 
This one is a hot topic of dispute - some cyclists prefer you to honk when you’re coming up behind them, others see it as intimidating.
I’m in the latter group
My local police advise that you should never honk at child cyclists unless they doing something particularly egregarious and an accident is imminent - for simply riding “normally” and competently on the shoulder, a honk is more likely to startle the cyclist and cause an accident than keep anybody safe.
Of course! When you lock The doors.