This fascinating Washington Post story on African American sign language contains an interesting tidbit: American Sign Language has much more in common with its French counterpart than forms of signing in other English-speaking countries:
Another widely held but erroneous belief is that sign languages are direct visual translations of spoken languages, which would mean that American signers could communicate fairly freely with British or Australian ones but would have a hard time understanding an Argentinian or Armenian’s signs. Neither is true, explains J. Archer Miller, a Baltimore-based lawyer who specializes in disability rights and has many deaf clients. There are numerous signing systems, and American Sign Language is based on the French system that Gallaudet and his teacher, Laurent Clerc, imported to America in the early 19th century. “I find it easier to understand a French signer” than a British or Australian one, Miller says, “because of the shared history of the American and French systems.” In fact, experts say, ASL is about 60 percent the same as French, and unintelligible to users of British sign language.