Detecting automobile honking
if you wanted to make use of the fact that the horn is normally two horns at different pitches, you might consider a pair of tone detectors (using either BPF or parametric EQ or a Goertzel filter and squaring and LPFing the squared output) similar to DTMF.  you have to detect a minimum portion of the whole audio energy in the two tones and they have to be reasonably equal energy for the two tones.  that will identify one model of horn.  you would need other pairs for other horn models.  but you should be able to detect a single horn honk that is louder than the other din.

detecting a cacophony of horns is a bigger problem.

you might want to gather a collection of sampled horn sounds (as Sagie suggested) and run a MATLAB spectrogram on each to see what you might have.