The horn is an essential part of the rider’s armoury. As a matter of law you cannot use your horn between 11pm and 7am in a built up area, but if I thought the use of my horn would stop me getting involved in a collision I would not be checking the time first for a non-endorsable offence.
The horn should not be used as a rebuke and, for what it’s worth, I think that is good advice. Sounding your horn after a danger has passed is more likely to get a car driver panicked, angry or otherwise discombobulated – blaring away serves no purpose apart from making the rider look like an aggressive jackass.
So, why the horn? The hearing part of the brain responds much more quickly than the visual part of the brain. This is because the human eye creates a 3D image and misses much of the detail. This is evolutionarily programmed into us. We are programmed to notice what could hurt us (predators) or what we can eat. The rest is background and the brain does not want to waste space on processing visual signals that do not lead anywhere.
Regrettably that is where we on bikes figure. We are a small object moving usually in a straight line and neither threat nor food, but if we are the same small object moving in a straight line with a 115dB horn blaring we become perceived. It is for this reason sports referees use whistles not flags, you hear a mosquito long before you see it and sprint races are started by a gun, not a flash. On a neurological level hearing is much less complex than seeing and is processed a great deal faster. It is our fastest sense, quicker even than touch. We also have an alarm reaction to a loud and frightening noise.
Your own experience will back this up. If you hear the blast of a horn your immediate reaction is to stop. Sound is non-directional. You can be facing in the wrong direction and hear, but with forward facing eyes if you are looking the wrong way you will see nothing. If you are looking in partially the correct direction you may perceive movement but not in detail. A loud horn is not obscured by glass, metal and airbags.
So if you fear a car may emerge into your path or pedestrians are getting kamikaze don’t be shy to use your horn and the louder the better to pierce iPods and in-car entertainment – a loud horn may cause a pedestrian or driver to freeze but that is more predictable than having them carry on in their own sweet bubble of ignorance.
Loud pipes may indeed save lives, but they result in a lot of irritation and police attention. A 115 decibel horn makes noise when it needs to and could save your skin without breaking any laws. So rather than flash, get horny.