There's a route I usually do that has some specific parts that look uphill but ride and feel as flat, and some parts that look flat but feel as uphill.
Besides this being an optical illusion, which is my pet theory so far, are there any other possible causes for this phenomenon, like type of road, for example?
Our sense of gravity/balance is not very precise on its own and it is combined in the brain with visual clues and other information. For example, it is really difficult to hang a picture straight on a large wall if you don't have a level tool or can align it parallel to the floor/ceiling. There are a few effects that can distort your sense of what is "horizontal".
The Ames room is an optical illusion where you think lines to be perpendicular where in reality they aren't. If the surrounding hills, trees etc. form lines at certain angles (e.g. a ridge on the horizon that your brain expects to be horizontal but that really slopes), you can get a similar effect in nature. Wikipedia explains it in relation to "magnetic mountains" or "gravity hills":
Ames' original design also contained a groove that was positioned such that a ball in it appears to roll uphill, against gravity. Richard Gregory regards this apparent "anti-gravity" effect as more amazing than the apparent size changes, although today it is often not shown when an Ames room is exhibited.
He speculates that "magnetic hills" (also known as "gravity hills") can be explained by this principle. For a magic mountain at an unnamed location in Scotland, he found that a row of trees form a background similar to the setting of an Ames room, making the water in a creek appear to flow uphill.
Also, your brain adjusts the sense of balance over time. You often get this when you cycle up a long steep hill followed by a less steep road with very little inclination. The flatter road may look like actually going downhills (even though it's still uphills) because on the steep climb your brain adjusts and your sense of what is "horizontal" gets tilted a bit. Often, its only when you turn around that you actually get a real sense of the inclination.
The most common cause in my vicinity is headwind / tailwind.
However, there are places where the background gives subtle clues indicating an inclination (or lack thereof) at odds with gravity. This is sometimes known as a 'gravity hill' or 'magnetic mountain', or something similar. There's a long list of places in the Wikipedia
These external clues can be of varied nature - trees growing at an angle to the ground due to prevailing winds, stratified rock formations with a slight slope or long sloping hills in the distance.
I have experienced three types of this kind of missperception where visual clues and psychology play a big role:
An almost uniform slope but curvy road designed for cars. Obviously, when cycling you go on one side of the road. This, combined with the banked turns makes you feel like turns towards one side are much easier than opposite turns. Naturally the banking alters the actual slope angle at the sides of the road, but also the perspective of the road against the presumed vertical pines around it accentuates the difference between the expected effort and the actual effort of riding each next curve.
Another case is a short stretch of road that first goes down and then goes up. When traveling by car on that place, you feel that the first part is a steep descent and the second part is a steep ascent. But when traveling by bike, the "feel" is not as much. First in the descent you don't get as much "free speed" as you expect, so you "pedal hard" to help overcome the following ascent, but when you actually climb, you feel like if you where 2 or 3 gears lower than you are (effort wise). In this case, the lack of a visual reference of a horizontal line in the traveling direction fools the brain.
The third case is a coastal road that leads to a climb but before the beginning of the climb, there is a diversion that goes down to a boating club. Since both the main road and the diverted exit go along for a few meters, when you go over the main road you visually feel like you are ascending, but with very low effort. I.e. the brain is fooled into thinking the climb starts sooner then it actually does.
However, I have also experienced "temporal" delusions that Include wind effects, bike condition and physical condition.
Wind effects are pretty obvious. I know places where winds regularly change during the day, so you may have better luck climbing in the morning than late in the afternoon.
Sloppy maintenance on the bike can lead to more perceived effort, thus leading to feel a flat like it was a climb. At least in my case, I do my own bike maintenance, but sometimes I'm less careful and omit some revisions. So I take my bike (fooling myself) thinking it is in perfect working order while in reality, hubs, chain or derailleur pulleys needed deeper cleaning...
Finally, something that has happened to me a lot, is that riding a route, I feel pretty energetic the whole time, but the truth is I'm more tired by the end of it, so the sloped in the last part of the route are perceived steeper than they actually are. With flats it means you'd feel them like climbs. This can be proved with large looped routes. If you start in what usually is the middle point of the route, at first you will feel like the climbs have been flattened (respect to your memory of them) and at the end, you'll feel the opposite.
