Sounds impractical and stupid for a game like INS:S, but I believe a car horn has its uses.
A car horn can alert allies to get out of the way, call allies that their ride has arrived, and to humorously alert an unexpecting solider that's going to turn into road kill.
[EDIT: From what I've been told, there is a way to honk the technical's horn, but I believe that it's not widely known and that the key binds from the Settings menu doesn't show that, so developers should just add in a key bind option to edit the controls to honk.]