Approaching a work zone you see an orange PREPARE TO STOP sign with an amber light on each side, alternating side to side, and a tab under it reading WHEN FLASHING. The lights are flashing. What does that tell you that the sign alone would not?

Correct answer: B. That a stop is required right now — the flashers are only activated by something that will actually require you to stop

That the stop is happening now, not merely possible. Book 7 requires the flashers on a TC-20A sign to be used with the WHEN FLASHING tab, and sets a strict condition on them: "the amber flashers used with the TC-20A sign must not continuously flash, and only when triggered by an event or situation that will require a stop." So a lit pair of flashers is a live signal that traffic ahead has stopped or is about to be stopped — often triggered automatically by a detector where the back of a queue is hidden. The plain PREPARE TO STOP sign works the same way over longer periods: it "must be covered or removed in periods when there is not a high expectation or certainty of having to stop", so neither version is scenery left up after the crew has gone.

Source: MTO Driver's Handbook: Ontario Traffic Manual, Book 7 — Temporary Conditions (April 2022), Section 4.2.8.5 — Prepare to Stop Signs (TC-20, TC-20A, TC-20At)