[derobert]

If you make the horns quiet enough for the nearby pedestrians, they'll be too quiet for other drivers to hear (since they're further away and the car is blocking a lot of the sound). And the driver might have the radio on, or even if not it's got to overcome the road noise.

If you make it the painful for the driver to use the horn, they just won't (which would maybe be better overall in cities, but then just remove the horn entirely). Or the sudden loud noise will surprise the driver, or make them instinctively cover their ears (i.e., remove both hands from the wheel). Neither of those would be good for safety.

So, unfortunately, not an easy problem.

In suburban and especially rural areas, the horn needs to carry further and there a few pedestrians. But I guess you could adjust volume by GPS location.

[kevincox]

Maybe as well as an audiable signal a radio signal can be emitted. Cars hear this radio signal and play a horn inside to the driver. Eventually when this is supported by all cars the outside horn can be made quieter (just for nearby pedestrians) as penetration through a different car that is playing loud music is no longer an issue.

Of course the depreciation period for this is enormous.