I think cars should have two horns. The normal, loud horn, for when you need to use the horn, then a second, nicer sounding horn. This would be a standard, pleasant sounding thing that every car would have. You would use this when you're sitting behind someone at the light and they don't notice it changed.
The normal horn is so loud and shocking that the person in front gets embarrassed AND thinks you're an asshole. It's hard to just do a short, little, nice toot on the traditional horn. I have to press so hard that it ends up being a huge blast of horn. By default, they would think I'm an ass.
The nice horn would make life more pleasant. Like a remote controlled "excuse me."