Actually they cruise more around and up to 16 miles an hour or at least that is what was said on the special I watched on Sunday night on Animal Planet about Black Mamba's. It was one of those Mutual of Omaha specials and was pretty good. Showed a clutch hatching, at birth they are about 2' long.

And they do hood. And they can get as long as 14' and rise up high enough to look a grown man in the eyes. Very nasty critters