I have not heard much pleasant said about Vietnam's traffic patterns and driving. However, I can't say much for the drivers around here in the United States, either. Several times, I've been very nearly hit by people that were talking on their cell phones and drinking a coffee with one hand while eating a slice of pizza and beating their children in the back seat with the other. People just aren't looking at what they are doing. That's the primary problem with cars. They are little rooms that people hang out in. And with vehicle safety the way it is these days, as long as you're wearing a seatbelt, you're pretty much going to walk away from any traffic accident with only scratches. There isn't much incentive to know what's going on outside your little world. And anyway, bicycles and the people on them are just an impediment to the soccer practice or rabid consumerism or wherever it is you are driving your minivan full of brats.
Anyway, here's how it works. There's a bottle of compressed air which attaches via a tube to the handlebar-mounted horn. I have my bottle in a third, under-the-down tube cage, but it comes with a velcro thing so you can strap it on wherever without having to use up a bottle cage. The air is compressed, so you press a button on the handlebar part and it unleashes 120dB of bicycle justice, as long as there is air in the bottle. There's also a little volume knob, if 120 is 'too much justice' but I always keep it on full blast. Once the air runs out, you can refill it using any bike pump. I keep forgetting the 'maximum recommended pressure' to refill it but I keep upping whatever number I thought it was last time by five or ten "just to be sure." Anyway, the higher the pressure, the louder the horn, so bring it on.