Approaching a pedestrian crossover, you pass a sign reading "NO PASSING HERE TO CROSSING". What is it doing, beyond restating a rule you already know?

Correct answer: B. It shows where the 30-metre no-passing zone begins and warns of the crossover ahead

It marks the exact point where the no-passing zone starts, and it warns you the crossover is coming. Book 5 describes the sign as prohibiting "passing within a 30 m stretch upstream of the pedestrian crossover," and lists two jobs for it: it "provides advance warning of the upcoming PXO" and "indicates to drivers that passing is prohibited from the sign to the crossing location." The 30-metre rule applies whether or not the sign is there, but 30 metres is hard to eyeball at speed — the sign turns an abstract distance into a line on the road. It is a required component at every pedestrian crossover, so its absence is a clue that what you are approaching is not a crossover.

Source: MTO Driver's Handbook: Ontario Traffic Manual, Book 5 — Regulatory Signs (December 2021), Section 4 — NO PASSING HERE TO CROSSING Sign (Ra-10)