I want something that does not sound like a bike (e.g., a ding-dong bell), or an emergency alarm (e.g., the AirZound), and that gets over the noise of city traffic. A car horn is designed for that, and a driver would react to a car horn-like sound in a more defensive way (e.g., by stopping, not changing lanes, not opening the door abruptly) than if he/she would hear a ding-dong or a compressed air noise (which could be anything, not necessarily a large-enough vehicle approaching).