The horn is a component found in (almost) all cars, and it's no exception in the Carmageddon series. Although it doesn't play an important role in gameplay, it still has its utilities.
The main use for the horn is to scare pedestrians. Any ped that walks by will get scared by the horn and will start screaming in panic. But, seeing as sounding the horn makes pedestrians run, what's the use for it? Well, when a pedestrian scares itself, it'll stop still for a second and only then will it run. This is the opportunity to smack it! Although, this is very hard and nearly useless to pull off.
Yet, the running itself can be useful. Suppose you have the stupid pedestrians powerup in combination with the turbo pedestrians powerup in a tight space. Seeing as they are stupid, they'll hardly move, but sounding the horn will make them run everywhere, and they might end up running against the car. This is practically impossible or improbable in a real race, but it's a good idea to cheat and have fun with.
The horn is used to scare calm pedestrians. Peds under the effect of the pedestrian Valium or stupid pedestrians powerups will walk without a care. Yet, sounding the horn will make them snap out of it.
Carmageddon TDR 2000
In the controls menu, the horn is known as "siren". Yet, it doesn't seem to do anything if used in a race. Maybe it sounds the siren on the police cars...[confirmation needed]