Some motor vehicle drivers seem to get particularly intrigued by why, sometimes, they find people on bicycles using what they call "the middle of the road." These people seem to have never used a bike, so I decided to write this to help them find the answer.
The phenomenon seems more pronounced when there are dedicated road options for bikes. Most bike riders hate sharing the road with motor vehicles, so if they choose not to use a dedicated bike path, it means it is probably bad, as in: having bad pavement; not being well maintained; not taking the most direct or preferred route; or not being long enough to be worth the effort of getting in and out—which is aggravated if the access is bad. Sometimes they don't even know there is a dedicated path.
When the only option is to share the road with cars, in most cases it is expected that bikes stay on the right side of the road, and sometimes there is a painted line marking a bike lane. The problem is, the side of the road is the place where you find the most debris and obstacles, so sometimes cyclists will actively move further from the side to avoid punctures, dirt, other damage to the bike, or even a crash.
When in groups, cyclists often stay side-by-side instead of in a single line, which can be upsetting because it takes up more space. Group rides can have complex positioning dynamics, and it can be beneficial to have that formation for performance or safety reasons. Often they just want or need to be able to talk to each other, and being in a single line makes it hard.
During intense training situations, cyclists can overstrain themselves a bit, and one of the effects is blurred or narrowed vision, which obviously impacts their sense of space.
In some roads, or in certain conditions, it's just unsafe to be on the side. High-speed descents, for instance.
Sometimes cyclists get distracted by the landscape, other vehicles or people on the road, or they just get lost in their thoughts. I also feel that any road or lane instinctively pulls you towards the middle if you don't pay strict attention at all times. No one is perfect.
I want to give special attention to the cases where the cyclist actively stays in the middle of the road for the purpose of holding their position and keeping other vehicles behind them from overtaking. Many motor vehicle drivers seem to have what I want to call Vehicular Dominance Syndrome. They believe they must overtake a slower vehicle, especially bicycles, no matter the conditions, as soon as possible.
Drivers that suffer from Vehicular Dominance Syndrome will overtake at high speeds with little available space, even if there's a traffic light, roundabout, or stop sign just ahead, or if the typically slower vehicle is actually faster on that particular road—on technical descents, for instance. Cyclists who are used to this kind of behavior will stay in the middle to prevent this kind of overtake that would make them have to brake more than they usually would have to, or even stop where they usually wouldn't, because a car overtook them and is now in the way.
The one with the larger footprint is you. The only reason a 40cm-wide vehicle is in your way is because you're six times wider.
You are used to slowing down or stopping for all kinds of things on the road: stop signs, red lights, heavy vehicles, construction, pedestrians, roundabouts, emergency vehicles, and so on. Why does someone on a bike trigger you so much?
Your vehicle likely weighs 100 times more, and takes up 20 times the area. You're in a comfortable seat in a climate-controlled metal box with all kinds of safety features and with inhuman horsepower available at the tip of your foot. You have to slow down for a moment because someone on a bike is in front. Look at yourself. Isn't it a bit disproportionate to get upset?
Lastly, try using a bike next time, and you won't have this problem.