Car horns, truck horns, train horns, emergency vehicle sirens, all standard issue for the road.
Do you think sound devices are necessary for bicycles in traffic? If not necessary, helpful in any way? Do you use one, or would you consider it? Have you had any near-misses that underscore the value of a road-worthy sound device, such as would penetrate a rolled up car window with driver listening to music or on the cellphone?
If you use a such a device, what is it? How easy is it to use? Do you have any better ideas for a sound device than what you currently use?