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I would worry about unintended second-order effects of that. If there is a danger, I do want a driver to honk quickly and without hesitation. Might imposing an external after-the-fact judgement on whether it was neccesary or not, with a very stiff fine, cause people to hesitate in situations where they shouldn't?

In the US, this varies per state. In most states, though, it is illegal to "unnecessarily" honk your horn. But as the sibling commented about the UK, it's rarely enforced and is at the discretion of the officer.

I don't think most people know it is illegal.

In the UK the law is similar but not enforced, I don't think the penalty is that great either?

Comparing it to talking/texting on a mobile phone while driving, where the penalties are moderate and there is some enforcement, attitudes are changing - very slowly though.