Horns are the most overused device in the world. The only thing a horn does it make you pay attention(which you should be already) and/or annoy you. If you need it to make you pay attention while driving then I think you should consider being a passenger instead. I, for one, never flinch at the sound of a horn anymore as it's such a daily occurance. In my own car I can't even hear a horn unless i'm idling due to the tone/loudness of my dual exhaust

As for signing in the car... I do it on occasion and I feel its none more distracting then a hearie driving on the cellphone and changing radio stations. Driving is all about eyes, feet, and hands... none of which deafness and/or hearing effects.