Mark Rober (previously at Neatorama) customized his car horn to make three different sounds for different purposes. One is a "courtesy honk" that doesn't sound nearly as rude as a regular horn. Wait until you hear the others.
Rober shares how he did it, too, with lots of links at the YouTube page to the parts he used, and even a WAV file for the sound. -via reddit
and I agree something like this ought to be a standard equipment item