California law provides,
(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.
The Ninth Circuit upheld this law earlier this month, against a challenge brought by a driver who wanted to honk her horn as an expression of support for political process. The court, in Porter v. Martinez (opinion by Judge Michelle Friedland, joined by District Judge Edward Korman, with a dissent by Judge Marsha Berzon), generally reasoned that the law was a content-neutral restriction on expression, and is narrowly tailored to the substantial government interest in traffic safety:
