A large orange LOW BRIDGE AHEAD sign appears on a highway carrying roadworks. Why is it placed where it is, and what does its presence tell you about the structure?

Correct answer: B. It sits before the last exit a tall vehicle can take to avoid the bridge, and means the clearance is under 4.5 m

It is placed to give a tall vehicle a way out, and it means the clearance is genuinely tight. Book 7 requires the sign to be installed "approximately 400 m in advance of the last exit that trucks can use to avoid low clearance in a construction area", and to be used "only where the clearance of the structure is less than 4.5 m." That placement is the whole design: past that exit an over-height vehicle has no escape, so the warning has to come before it. The manual also notes that in a work zone "as work progresses, clearance may change frequently", which is why these are orange temporary signs — the black-and-yellow LOW CLEARANCE signs are for a permanent or semi-permanent condition.

Source: MTO Driver's Handbook: Ontario Traffic Manual, Book 7 — Temporary Conditions (April 2022), Section 4.2.8.5 — Low Bridge Ahead Signs (TC-33, TC-33A, TC-33B)