I think it's interesting how our brains determine the size, location and speed of objects based on the visual cues around the object. This helps explain why car drivers often "don't see" a motorcycle.
The rest of this post is a lot of science and psychology so if you don't car about either, please ignore!
The web site below has a good overview and good examples of depth perception.
This is my explanation on how all this applies to how drivers see motorcycles:
Smaller objects are interpreted by the brain to be farther away than large objects. If the driver's brain expects only car size vehicles on the road, the much smaller motorcycle is perceived by that person's brain to be very far away. Our brains often chose to ignore information that is not important to focus on what is, especially in situations where there is a lot going on that must be interpreted and reacted to, in order to stay safe. For example, at an intersection the driver is looking for approaching vehicles, their speed and how it is changing and clues that other vehicles intend to drive straight or turn. The small motorcycle is interpreted to be a normal size car/truck that is a far away and therefore can be safely ignored by the driver's brain.
Our brains are constantly ignoring things, like the noise of a running refrigerator, the weight and feel of our clothing throughout the day and a constant light source.
But, our brains also uses the amount of detail (texture) to determine the distance of an object. If you can see a lot of detail on an object, compared to the background, the brain determines that object to be closer. Bright clothing, lights etc make the motorcycle stand out against the background, and provide more details and "texture gradient" to the driver's brain. More details = the cycle is closer. I would also guess that shiny helmets and paint gives more depth perception clues than flat colors.
In addition, once the brain "realizes" the approaching vehicle is a motorcycle and knows it really is much smaller than cars or trucks, it "recalculates" how far away it is, realizes it is closer and then pays attention to it. Hopefully, this experience also helps in the future as the driver's brain realizes that small vehicles are also on the road and is "looking for" them.
Of course, all of this is happening almost instantly.
This process is conscious and unconscious, based on real knowledge of the world around us, our attentiveness at the moment and our experiences. The brain's interpretation of what the eyes detect is why new drivers need lot's of experience to drive safely in traffic. And the "slowing" of our brain that comes with drug or alcohol impairment, aging, or making something else the focus of our attention makes driver's less safe.
It also means that we can improve our ability to be seen and recognized as being close with anything that separates us from the background.