The ordinance does not attempt to regulate the “content” of horn-honking. Rather, it prohibits all horn-honking, except in cases of imminent danger, regardless of the user's intended meaning.
The ordinance also leaves open alternative means of attracting the attention of a police officer. It remains for a motorist to flash his lights, signal with his hands, or call out. The plaintiff, nevertheless, asserts that his message was stifled because he had no alternate means to relay his message. However, in the same breath, he admits that his message was not, in fact, stifled; a police officer who acknowledged his hand signal proceeded to ignore it.
Thus, the ordinance neither discriminates among messages nor limits the expression of any particular message. It is based on the manner of expression, not on its content.