Halmyre - I love the idea, but I already do something like this. When walking, I carry a "tornado" whistle(about twice as loud as a referees whistle), and I have a small compressed air horn that fits in a pocket(or mounts on the handle bars) when bicycling. Sweet thing about the air horn - I made it using some stainless steel piping and a schrader air nipple(i can recharge it for free) like the ones on car wheels. The horn is just pulled from a standard store bought marine air horn. I'm working on making a more compact horn now.