This mod adds horns to cars and tanks.
Both vehicles have their own set of two sounds, a shorter one and a longer one, which can be bound to different keys. ("H" and "SHIFT+H" by default)
Honking also attracts enemies in a 60 tiles range from the user. This range can be tweaked in-game via the per-user mod options. (Note: biters normally attack when within 30 tiles from you)
Likewise, the enemy attraction mechanic can be disabled altogether.
If you feel the horns are too loud or quiet, there's a volume multiplier on line 1 in data.lua (default is 1.2)
Here is a short video showcasing the mod's features:
Check the thread on Factorio Forums for more information.