About 150 metres ahead an orange TEMPORARY BRIDGE sign carries a tab showing a speed. What is that speed, and what should you do about the bridge itself?

Correct answer: B. An advisory speed for the bridge, set by the highway authority — slow to it well before you reach the structure

It is an advisory speed, and the bridge needs you well below your normal pace. Book 7 says the TEMPORARY BRIDGE sign "must be used to warn of a temporary bridge ahead, on which the traffic speed is severely reduced", installed "approximately 150 m in advance of the bridge". The tab is optional and advisory: it "is not mandatory but may be used for an Advisory speed, to be determined by the highway authority." That is a real distinction — an advisory speed is the speed the road authority judges the structure can be crossed at, not a posted limit — but it is not a licence to ignore it. A temporary bridge is narrow, often with a rough joint at each end, and 150 m is not much room to shed speed, so start slowing as soon as you read the sign.

Source: MTO Driver's Handbook: Ontario Traffic Manual, Book 7 — Temporary Conditions (April 2022), Section 4.2.8.5 — Temporary Bridge Sign (TC-32) and Temporary Bridge km/h Tab (TC-32t)