I'm also not quite sure on why exactly it is so complex to distinguish between objects (including vehicles) on the road and ones above/next to it. In the case

I'm also not quite sure on why exactly it is so complex to distinguish between objects (including vehicles) on the road and ones above/next to it. In the case they're talking about here, it's because to do so you need to predict where the non-visible road surface is going to be. Consider travelling up a continous, slight incline. Precisely at the crest of the incline is an overhead gantry sign, positioned such that for an observer travelling up the incline it is located directly in line with their current direction of travel. The only way that you "know" that the road actually dips under the sign rather than the sign being on the road surface is experience.