Driving along a through road, you pass an intersection warning sign with a tab beneath it reading HIDDEN INTERSECTION. You can see the side road perfectly well. Who is hidden from whom?

Correct answer: A. You are — the tab warns that drivers waiting on the side road cannot see traffic approaching on your through road

You are hidden from them. Book 6 is explicit about a word that reads backwards at speed: "the word 'hidden' refers to traffic on the road with the designated right-of-way, which is hidden from traffic on the side road." The tab is used "to warn drivers on the through road that side road traffic under stop or yield control does not have an adequate view of traffic approaching on the through road." So the sign is not telling you that a side road is about to appear from nowhere — it is telling you that a driver waiting at that stop or yield sign may pull out into you in good faith, because they cannot see far enough up your road to know you are coming. Cover the brake and be ready for exactly that.

Source: MTO Driver's Handbook: Ontario Traffic Manual, Book 6 — Warning Signs (July 2001), Section 3 — HIDDEN INTERSECTION Tab Sign (Wa-18t)