A yellow diamond sign shows a school bus symbol with an arrow, warning of a school bus entrance ahead. What does the fact that it has been posted at all tell you about what is coming?

Correct answer: B. That your view of the entrance is 250 m or less — the sign is only used where you could not otherwise see the buses in time

That you will not see the buses in time without it. Book 6 restricts the sign to genuinely blind locations: it "must be used only where the driver's sight distance to the location where buses enter or leave the main road is 250 m or less. If the school bus entrance location can be viewed from a distance greater than 250 m, it is assumed that the driver has sufficient time to see the hazard and stop." It also explains what makes the spot hazardous: buses "entering, leaving, or crossing the road" where "an unusual manoeuvre by the school bus, or a physical condition (e.g., inadequate sight distance, steep grade) presents an uncommon degree of hazard." The arrow tells you which side the entrance is on, and the symbol replaced the older text-only SCHOOL BUS TURNING sign because a symbol takes less time to read.

Source: MTO Driver's Handbook: Ontario Traffic Manual, Book 6 — Warning Signs (July 2001), Section 7 — SCHOOL BUS ENTRANCE Sign (Wc-10, Wc-10L, Wc-10R)