This system has two parts, the receiver /driver
and the aimable transmitter / horn trigger.
A diagram on the dashboard has 6 buttons representing
cars around you. Push the button in the position that
approximates the car you hate and a signal is sent to
giving them a horn blast in the driver's
compartment that doesn't bother the cars around it.
You'd probably have to have a button representing the
behind you so you can give the guy who honked at you a
Alternatively, you could have an array of cameras on the
dashboard sense the direction your finger is pointing and
the position of your thumb. When you point at a
car and drop your thumb in a mocK gun shooting gesture,
honk signal is sent to that car.
Why would people leave their receivers on to be
subjected to a horn going off in your car at the whim of
some jerk? Because the system is
wired such that it's either all on or all off. You can't just
targeted honks at people without being able to receive
targeted honks as well.