Growing up, many deaf students were forced to learn how to speak and lip read. As adults they are able to speak clearly and be understood, but many of them prefer to "turn their voice off", what is the reason they prefer to sign rather than speak?

Because lip reading is HARD

Explanation:

You can't be 100% accurate lip reader. Since someone is deaf they cant hear, so all they can go off of is your lips. Don't you think it would be easier to just look at someones hands. Also I think that a deaf person would prefer to use sign languages because other people around them might be deaf too. But maybe they would talk if someone they were talking to did not know sign language.

can you show the map , i'll be able to update my answer