What is the most difficult thing to estimate during an overtaking?
It is very difficult to judge how fast a car coming straight towards your own car is driving. The reason being that there are no clear reference points between which you can “measure” their speed. If you are looking at a car from the side, it is much easier to judge its speed.
“If you are heading towards another car, you have to remember how difficult it is to judge where you will meet.” (Driving Licence Book, page 87)
Note that the other alternatives are fairly bad, i.e. these are things that are relatively easy to judge.
More examples of explanations: