Quote Originally Posted by Ducatista
Also just wanted to mention it can be a really bad idea to use a horn in some circumstance.

I did it once when someone pulled out on me and they braked in front of me.
Their car brakes were superior to my little bike at the time and I crashed into the back of them.
That's my fault because I didn't anticipate how they would react to my horn (emergency brake in front of me).

I can think of two near misses I've had in recent years where someone has pulled out in front of me.
On both occasions i did NOT use the horn and both times the vehicles moved allowing me to avoid them.

One of them could have been quite a serious T-bone.
A van pulled out on me from the apex of a bend and stopped in the road.
I put my brakes on but did NOT use the horn.
He moved on probably totally unaware of my presence and I was able to make a last minute avoidance manouvre behind the back of van as he'd moved off.
If I'd of used the horn in that situation he probably would have stopped blocking the entire road and looked for the danger and I would have T-boned the van causing death or serious injury.

So the horn needs to be used judiciously and you need to consider how other motorists are likely to respond and what effect that has on you.
In general they are going to spend a few potentially crucial seconds looking for where the sound is coming from and they may think they are helping by braking when the opposite may be true.

Completely agree. All of which proves that there's no one rule for all circumstances. Its a question of judgement. I tend to use the horn well in advance of a hazard, rather when the hazard is actually close... But each case is diffeent as Ducatista's experience shows..