The inattentive motorist misses you because you're not part of the herd. There's a whole chapter on eyes-forward predatory lizard-brain instinctual shit in either the lee parks or the hough book on this, and it's been heavily discussed. Make yourself SEEN, and you'll be seen. Adding a horn is just mitigation of potential exchanges of rapid inertial changes after you and the supposedly inattentive drivers (sometimes that's very true, sometimes it's just not) have both fucked up to some degree. Your not being seen is often WAY more their fault than theirs, but it's 100% your responsibility to deal with the aftermath. Be seen. Leaving being heard as a last-second "oh shit, everything else has failed" Hail Mary.

^What he said

I agree that beeping after the fan has turned brown is pointless.

I've had mixed success with preventative beeping. Particularly when someone is merging in on me on a multilane road or highway--they usually go back to their lane or at least stop merging long enough for me to get safe.  But there was one time where a pickup, ironically with a Harley bumper sticker, just kept merging without paying any attention.

The particular instance that prompted my question seemed just right for a preventative beep to work.  A cager on a side street was engrossed in something the other way while inching forward.  This caused me to assume the cager was going to pull out onto the highway without looking.  I saw that I could safely evade by swerving into the oncoming traffic lane and getting back before I put oncoming traffic to any inconvenience.  But I gave a good beep expecting the cager to stop moving forward and to look in my direction.  Didn't happen. 

The cager never once turned to look in my direction while pulling out.  There was no mishap because I was prepared to maneuver.  But if I were operating a less maneuverable vehicle there likely would have been a crash.  I'm wondering whether a louder horn might have gotten through to the cager?  Even so, would the cager have stopped moving?  Chornbe, you may very well be right that the cager might not have looked for a motorcycle even if the cager had been prompted to turn and look in my direction due to a loud horn.  I suspect the same thing happens with bicycles and pedestrians being overlooked.

I agree that high viz is helpful, but it doesn't make a difference when the cagers aren't even looking in your general direction. Same with flashing the high beam in this situation.

No one thing guarantees safety.  Yet a multitude of approaches may increase the odds of having a good day.

Flashing high beam says “go ahead”, 100% of the time around here. Having a modulator around here only promotes the pulling out in front of you. I’m so not a fan.

I've heard of flash meaning "gord head own" but never lived where that was a thing. I've also never ridden somewhere that I could point to an incident where my modulators ever did anything other than I intended them to.

.....  As for sitting at an intersection where everyone is hesitating to go for whatever reason, I'm used to waiving the other person that they have right-of-way.

 I wave the others on, regardless.  I'm on two wheels and not going to get in a pissing match over 'right of way' even though the laws say yield to the vehicle to your right.

The other day, I saw a several guys not see the Fire Chiefs bright red suburban with sirens and lights, followed by 2 bright red fire trucks with lights and sirens, followed by a bright red 50 foot hook and ladder fire truck with lights and sirens wailing.  One of the guys was so impatient he floored it and turned between the 2 fire trucks instead of waiting 4 seconds.

We have no chance.

what he said ^ 

one of the first things my father taught me when I started riding on the street was " if you want to know what they are going to do watch their front tires, not their faces". After I T-boned a car at 55MPH because I was sure I had eye contact with the pretty blonde I started following his advice.

Ok Bounce, you lost me.  What does "gord head own" mean?

Local dialect for "go ahead on".

For me, flashing the lights at oncoming traffic is a way of warning folks that a LEO is hiding ahead of them.  And flashing the lights at traffic you're following means that you want to pass.  As for sitting at an intersection where everyone is hesitating to go for whatever reason, I'm used to waiving the other person that they have right-of-way.

That's how I learned it and how it's implemented in OK, TX, AR, LA, NM. By extension, it's also what seems to work while I've ridden all over the US. Local side streets in various villages might be different.

Some days I ride as if I'm invisible.  Other days I ride as if everyone wants to kill me.  It's not paranoia if they really are out to get you.

And I drive a grey compact car as well. I think it's been invisible at times, too.  I wonder whether I should find a car in highlighter yellow?  But if they won't yield to firetrucks … ?

Well you might try three different types of horns... and there's a diy!

https://youtu.be/lv8wqnk_TsA

An air horn is probably overkill, but a nice loud low-tone horn seems just the thing.
Mostly because 'beep beep' isn't a sound the automobilists respect, but 'honk honk' is.

This one makes you sound like you might scratch up their car:
https://www.aerostich.com/fiamm-freeway-blaster-horn.html
Probably cheaper on Amazon.

Thanks, everyone, for the input.  I've decided to quit dithering and ordered an air horn.  Denali Soundbomb with wiring kit:  https://www.revzilla.com/motorcycle/denali-soundbomb-compact-air-horn-and-wiring-kit

Basically, I have had occasions where sounding a horn was beneficial.  And while the air horn is extreme, I might be disappointed by something less extreme where I ultimately wind up wanting to replace the replacement. 

It's similar to ABS--just one more thing that may provide a greater margin in a particular situation.  It's not a panacea, but it does add a stronger arrow to my quiver (can anyone tell that I like fantasy novels?).  More tools in the ol' toolbag, perhaps? 

Had air horns one 3 wings. Put a Stebels (dual tone) on my FJR and have never regretted it. Cleaner install. Just as effective.

Turning right from a stop, someone on that cross street and going the same way I was headed, completely ignored that I was now ahead of them on the neighborhood street and started to drive past me and squeeze me into the curb. I laid into my horns. Their windows were down. The driver hit his brakes and the passenger (nearest me) recoiled so dramatically, I knew they'd both not soon forget it. I suspect they thought an 60s Buick had appeared off their starboard bow.

I've used my Stebel (twice) this past week to wake up some idiot checking up his textos while the light had changed and all cars ahead had already gone.
For some reason he seemed pissed by that. LOL

Personally, I never use the horn.

I've always thought these were pretty slick for those who do.

Screaming Banshee horn and light modulator...

https://www.youtube.com/watch?v=dPuzTQFVoVI

Stebel Nautilus FOR THE WIN.  You may never need it.  Until you do.  I've had the "keep merging left without the mirrors" as I've reached their driver's door.  A quick flick of a button stopped that and had the driver trying to figure out where the semi-truck was.  I was in hi viz, but if they ain't looking, it does no good.

Exactly the same thing happened to me. I was on a 3 lane section of a 16 lane highway and noticed the car on my right in about the 2 o’clock position starting to move into my lane. There was a tractor trailer on my left. I had nowhere to go. I hit my Stebel and she swerved back into her lane. I guess I could have slammed on my brakes but that would likely have me crushed by the truck behind me.
The horn paid for itself with 1 use.