How is american sign language superior to spoken languages?

Sign language is not better than spoken languages (if it was, then every culture would communication through signing).

But sign language does have some advantages:
1) Sign language is silent and thus can be used effectively in war time scenarios to reduce detection by the enemy
2) Sign language is limited by sight as opposed to sound, meaning that you can have a quality conversation with someone on the other side of a room (or even the other side of a building) whereas sound quickly degrades and you need to be close to the person you are speaking with, especially in a crowded room.
3) Other people cannot 'overhear' a sign language conversation if the hands are not visible.

well jazz7216 it is similar the fact which sign language is a language but there is a slight difference well it is not ''spoken'' it is read and it is really hand movement