I’ll cut right to the chase here: I’m already going as fast as I can, and I’m positioned as far to the right on the road as I can safely be. So, if you honk at me in an effort to get me to change my behavior, I only have two options: move further left, or slow down. I’ll probably do both.
In general, save honking for emergency situations. Really. It’s just not gonna help. What I’m about to say applies to dealing with motorists, too, but I’m addressing it from the standpoint of cyclists, since that’s where I encounter it the most.
In general, if it seems like that cyclist in front of you is doing something stupid, there are 3 possibilities:
- they’re an idiot
- they’re an asshole
- there’s a good reason for their behavior
So, let’s address each of those in turn. Cars are loud. Really loud. And it’s not the engine that is noisy, so I don’t care how tuned up your car is. It’s the tire noise on the road, and the wind noise around the body. So if you’re doing more than 15mph, I’ll hear you coming. With my headphones cranked, and tucked under a windproof (and therefore severely-sound-attenuating) headband in Winter, I still hear most cars coming half a block behind me. It’s even more obvious at night, assuming you have your headlights on. So, given that, the cyclist that is unaware of a car behind them is an idiot. Either because they’re really oblivious, or because they have misjudged their awareness and made a poor choice (like turning headphones up too loud, or not accounting for poor hearing, or being heavily under the influence of some drug). Any cyclist that is so oblivious to their surroundings as to miss a moving car probably won’t notice a honking horn, either–or at least won’t realize that it’s directed at them. And, if they don’t understand why you’re honking–which is likely, if they really are that much of an idiot–then if they do realize you’re honking at them, you’re not likely to solve the "problem". It either will have no impact, or it will turn them into an asshole, a not-unexpected response to what they perceive as you being an ass to them.
Now, the next possibility is that the cyclist you’re honking at is an asshole. They know you’re there. They don’t care. In fact, they’re deliberately inconveniencing you, just because they can. I’m pretty sure that most motorists think that of most cyclists, in fact. It’s not usually the case, I suspect, but, so what? Even if it is, when was the last time that honking at (or yelling at, or threatening) an asshole–motorist or cyclist or pedestrian, on the road or on the sidewalk or in a store–caused them to become friendly and/or compliant? If it has any impact at all, it’s likely to make them more belligerent, and possibly cause them to go out of their way to be troublesome.
Which leaves us with the case where that cyclist that is "impeding your travel" is doing so for a very good reason. Maybe they can see something you can’t, what with being ahead of you on the road, probably higher up, and without any opaque metal bits around them to impede visibility. Maybe there’s crud–or a storm grate–on the road that you wouldn’t even think twice about in a 4-wheeled vehicle, but that is very dangerous on a 2-wheeled vehicle. Maybe they’re employing defensive-driving strategies to discourage you doing something dangerous. Regardless of the reason, if the cyclist has thought things through and is positioned on the road thoughtfully and intentionally, their response to your honking is either going to be to ignore you, or to become asshole-ish in response to your behavior.
Finally, there’s one other thing to consider: startling someone balancing on their vehicle (especially a bike, where the person outmasses the vehicle considerably, but even on a scooter or motorcycle, potentially) is a good way to cause an accident. And now they’re gonna be holding up traffic even more. And I’m guessing that, as much as you wanted that !@$#* bicyclist out from in front of you, you probably don’t actually want to be responsible for killing anyone. Not to mention you might be [at least partially] liable.
Me? I’m case 3. I’m very aware of where I am on the road, and what’s going on around me. I’ve been making my living behind the wheel for about 15 year now, so I’m pretty considerate of other vehicles, especially working vehicles. I’m not gonna hold up traffic if I can possibly help it. But I’m also not gonna apologize for doing so if that’s the safest thing for me to do. My response to honking may come across as being a jerk but, if you ask me, it’s the motorist who presumes I’m not already being the most-considerate road user I can be that is being the jerk. He’s started out assuming I’m out to get in his way. Well, my response to that is to show him how I would behave if that were the case–and, as I said, to make the only adjustments in my behavior that I safely can. That’s if I’m feeling ornery. Most of the time, I just ignore the honking, and continue on. I might even be understanding, recognizing that the motorist might well not understand how belligerent their behavior really is. But I still won’t put myself in a dangerous position in the process.
And if I’m really tired, I’ll just stay off main roads as much as possible, because I know I’ll be going slowly and holding up traffic. It’s not like I’m unaware of my impact on other road users. I’m also aware that motorists often have an exaggerated notion of how big that impact is. There’s a hill on my way to work with one narrow lane in my direction. I’ll claim that lane as I’m coming up the hill, forcing any motorists behind me to to slow down or go into the oncoming lane to pass. Which, despite the blind hill, they routinely do. Now, keep in mind–even if I move as far towards the curb as possible, they’d still have to cross the centerline in order to pass me. So, I’m not "causing" that–all I’m doing is making it very clear what their choices are, and hopefully discouraging a few from trying it. Because, as predicted, when we come over that hilltop and there’s a car coming straight at them, they swerve right, regardless of where I am. But, here’s the kicker: I’ve timed it. Passing me at the top of the hill, rather than waiting for where the road becomes 2 lanes again saves around 12 seconds. Passing me at the bottom of the hill (generally involving going straight through the light in a left-turn-only lane, to boot) saves around 20 seconds, maybe a smidge more. So, people who pass me on that hill would rather gamble on meeting an oncoming car as they crest the hill, than take 30sec. longer on their drive.
So, let’s summarize: if the person you’re honking at is an idiot, you’ll likely either have no impact, or make them actively try to get in your way. If they’re an asshole, you’ll probably make the situation worse. And if they have a good reason for their behavior, they’re not about to alter it just because you’re honking–except, if the situation permits it, possibly to get more in your way. And, in addition to the fact that honking at that cyclist probably won’t get them to change their behavior, you might cause an accident.
 Keep in mind that "something dangerous" often translates into something dangerous for the cyclist, even if it doesn’t start out that way. When a motorist crosses the centerline to pass a cyclist, and then realizes that there’s oncoming traffic, odds are they’ll either momentarily forget about the cyclist, or decide the cyclist is less of a threat. Either way, they’ll almost certainly swerve right, right into the cyclist, rather than risk hitting an oncoming car or truck. So, part of safe cycling is trying to make sure that situation never happens in the first place. In places where a motorist might try to pass despite poor visibility, you move left. Yes, this forces the motorist even more into the oncoming lane. But the effect of that is that fewer motorists will attempt to pass in the first place–it’s feels much riskier to move fully into the oncoming lane than to just shift a couple feet over the centerline. And, if someone does pass, and does need to swerve back into their lane, the cyclist is farther from the curb or edge of the road, and thus has more room to dodge without going off the road (and likely losing control). So there’s a reason that cyclists are more likely to be farther from the edge of the road, the narrower the road gets, and it’s not just to piss you off.