I was smugly thinking that my mini-Fox 40 whistle would do just fine for signalling someone if my car were to go over the side - until I realized that I might very well be injured and unable to use the whistle for any length of time.<br><br>GREAT point about the compressed air horn! Wal-Mart has them in a variety of sizes in the boating/sporting goods sections.<br><br>Thanks for your suggestion.<br><br>NP