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Until you get inside a city traffic jam, and then it becomes an acoustic indicator of something you already know: every square centimeter around you is covered by a car or a motorbike.

I'm a skeptical horns help with anything in modern Indian city traffic, but on the highway they can be quite useful, if annoying, especially on mountainous road when you don't have a good line of sight, but even just on a normal road with drivers who don't look on their rear-view mirrors.