You might be aware already (I see that you're an attorney), but at least in California, honking is illegal except for safety purposes. > (a) The driver of a mo

You might be aware already (I see that you're an attorney), but at least in California, honking is illegal except for safety purposes. > (a) The driver of a motor vehicle when reasonably necessary to insure safe operation shall give audible warning with his horn. > (b) The horn shall not otherwise be used, except as a theft alarm system which operates as specified in Article 13 (commencing with Section 28085) of this chapter. Honking when someone doesn't notice a green light seems perfectly reasonable, if technically illegal. But I'm glad we have that law (albeit not much enforcement), since most honking just conveys impatience and increases stress for everyone without communicating anything useful.