[MSM]

They're both invisible to a blind person.

[aetherson]

So are knives, a clearly dangerous object! Knives should be regulated to emit constant noise.

No, seriously, I think you do the blind a disservice. I don't think that blind people rely on, "Hey, did I hear something" to mean, "I can blithely cross the street without danger."

[hughlomas]

What a childish analogy, knives are not moving at high speed on every street.