If you're a Tesla owner, you might not have to open a window to let your intentions be known to passers-by. Elon Musk has teased plans to let Tesla cars "talk" to pedestrians through external speakers (presumably the ones added to comply with sound requirements in the EU and US). He didn't outline how it would work, but the feature appears to at least support canned clips -- Musk even hinted that you'll get to "fart in their general direction" Monty Python-style.
This may be out of the reach of many older Tesla cars, and it's not certain if you'll get to either speak directly to people outside or send automated speech (say, to warn that you're backing up). It could still be helpful both for convenience and as a safety measure, though. You could alert an inattentive pedestrian without taking the time to open your window, or deter a would-be intruder without either honking your horn or exposing yourself to added danger. And given what Musk said, we wouldn't be surprised if you could use this to make people giggle as you roll past.