Motion parallax is the phenomenon caused by the relationship of our knowledge of speed of objects to the time necessary for those objects to traverse our visual field. It is similar to relative size in that we have gradually acquired a storehouse of knowledge about objects and their speeds. For example, I was fishing in an oxbow lake one day with a friend, and a crop duster flew over us. One second the plane was there, and the next second it was gone. My response to that experience was that a plane had flown very close to me. Later that day, I looked up and saw a B-52 bomber flying by. It looked very small and took many minutes to pass.
Notice what I did not say, "There is a very small plane flying by very slowly." Instead, I knew that B-52s are much bigger than crop dusters, and also fly much faster. So, motion parallax is interpreted as distance. The crop duster was close because it appeared and disappeared quickly; the B-52 was far away because it took a long time to traverse my visual field. Think what my perceptions would have been if the B-52 had flown over me at the same height as the crop duster.
Motion parallax mistakes can have tragic consequences. Over 1000 people a year are killed at railroad crossings each year in the USA. Perceptual naivete is part of the problem. Most of us have only limited experience with trains. Trains are bigger than other moving objects with which we have experience. Basically, we do not realize just how fast trains travel; we underestimate their speed. Also, looking at a train coming right at you provides a minimum of speed cues. That is the reason that trains now sport two lights in front, instead of one. Often, the second light revolves to provide further distance cues, especially at night. Less tragically, but exhibiting the flip side of this phenomenon, is our difficulty in estimating the speed of objects smaller than our usual experience, motorcycles, for example. We tend to overestimate the speed of a motorcycle because it is smaller than a car. Tell that to the officers as they write you a speeding ticket. But, remember, the radar gun does not have our perceptual limitations.