AnFI - place yourself in a position where you can see an object in the near or middle distance clearly with an absolutely perfectly aligned reference point in the far distance.
Close one eye and ensure your objects are lined up perfectly. Now rotate your head - simulating the effect of panning a camera - and you will see a parallax error appear.
The camera, it's lens and its focal plane will move at a distance from the pivot (ie the human holding it) and not rotate simply around the focal plane. There is your parallax error (change in position of the observer) and there is the weakness in your speed assessment.
Your video has no such perfect position references and you don't know the focal length of the camera lens (another variable causing distortion) or whether the camera was held at arms length or close to the camera operators eye - therefore you cannot calculate the amount of parallax and your speed assessment is pure guesswork.
Simple enough really
Btw, the oaf in the playground worked that out sitting in a café with a large mocha
