How about a bullhorn with a record function, that way you just say "Stay to the left please" just once, then either press a button every time you want to have the bullhorn repeat what you said OR have a setting where the bullhorn constantly repeats what you said, you can adjust the time delay in between repititions via a dial on the side.

Just for fun, add a few switches to tweek the pitch of your voice in case you don't like how you sound when you hear you own voice (Ever hear yourself on a friends answering machine?). Add a Darth Vader voice modulator as well, why not?