The primary purpose of a vehicle (car, locomotive, etc.) horn is to warn other traffic units that something potentially dangerous is about to happen. And so, others must pay attention to the signal.
So, why not to tune the horns to dissonant intervals or chords (for example, several notes semitone or even quarter-tone apart, e.g. C natural, C half-sharp and C sharp sounding simultaneously), which are much more unpleasant to listen and so more "attention-payable"?
Maybe this is not a proper place for the question, but I've unfortunately not found a better one, since is more related to engineering and design (than to, for example, music).