Why not? Everybody would use it. A single loud blast of sound is not a dynamic way of communicating in an arena of our lives where there is great peril at high speeds. Car companies invest so much money into crash tests and safety bags, you would think they could add a second sound to the horn. A horn that would say “Hey, the light turned green.” and other not-so-urgent messages. The advantage of this, other than allowing you to alert somebody to the fact that they are sitting at a green light without feeling like a jerk, would be to give emphasis to the use of the louder horn. It would gain importance as the more urgent of the two signals and would therefore be less likely to be ignored.