Vehicle Features/Horns

Horns are a feature that appeared in all GTA Games, it serves only to add realism and makes the players honk around the city, there are also sirens, reverse beeps, jingle bells, special horns and musical horns throughout the GTA Games.

In 2D Universe games
In GTA 1, London and GTA 2, you can honk the horn by pressing TAB on the PC, the police cars had sirens instead of horns, and the Ice-Cream Van had a jingle bells instead of a horn, though they serve for no real purpose.

In 3D Universe games
In GTA III, Vice City, LCS and VCS, you can honk the horn by pressing H on the PC and the Left Analog Stick on the PS2, also, a feature, unique to large trucks were the Reverse beeps, for the purpose of realism, as large trucks in real life normally has those beeps, in GTA San Andreas however, this feature is missing, but horns are used the same way as the other games.

In the 3D universe games onwards, emergency may move other vehicles out of the road, which makes it easier when doing the Vigilante, Paramedic and Firefighter side missions, and again, an Ice Cream van had a unique jingle bells instead of a horn, except for GTA San Andreas, where you could horn normally if you hold the horn button, the jingle bells of the Mr. Whoopee is used for the purpose of selling "ice cream" for the Cherry Popper Ice Cream Factory

Also, unique to GTA San Andreas, Bicycles had a bell instead of horns.

In HD Universe games
In Grand Theft Auto IV, the horn functions remain unchanged from the 3D Universe games, the sirens still move vehicles out of the way, and there were still an Ice Cream van with jingle bells in the game, though the latter serves for no real purpose, the reverse beeps had returned to the HD Universe games, now almost every industrial truck has it, and unique to the Mr. Tasty, it sound like a "ding-a-ling" instead of "beep", In GTA IV there were cars that had unique horns, not found in any other vehicle in game

Different from the 3D universe, boats now have horns, louder than any car horn

Also in GTA IV onwards, railway vehicles also had horns, just like a real life train, they can be heard randomly, or in GTA IV's case, only during certain missions

In Grand Theft Auto V the horns have been expanded, now you can even modify your car's horn at the Los Santos Customs, in GTA V car horns were more random and could be different even if the car is the same

In GTA V, Tow Trucks had sirens on the top, but makes no sound at all and serves only for realism, also, the Airtug had a unique siren that makes a sound of a reverse beep, oddly, it can move vehicles out of the way, and in GTA V, there were 3 different reverse beeps on the Trucks

Also, Bicycles, again, had bells instead of horns