On a rural highway you pass an orange sign reading DO NOT USE RADIO TRANSMITTER. Why would a road crew care what is on your airwaves?

Correct answer: D. Blasting is under way with an electrical detonating system, which a transmitter could set off — a RESUME USE sign marks the far end

Because a radio transmission can set off an explosive charge. Book 7 says the DO NOT USE RADIO TRANSMITTER and RESUME USE OF RADIO TRANSMITTER signs "must be installed where blasting is being done, whenever an electrical detonating system is in use adjacent to a highway." A two-way radio or other transmitter can induce a current in the detonating circuit, so the signs mark out the stretch where transmitting is unsafe. They are placed generously: the first sign goes up "1 km in advance of the blasting area", and the RESUME USE OF RADIO TRANSMITTER sign is installed "1 km past the end of the blasting area" — so the all-clear is a sign you actually see, not something you have to guess at.

Source: MTO Driver's Handbook: Ontario Traffic Manual, Book 7 — Temporary Conditions (April 2022), Section 4.2.8.5 — Do Not Use Radio Transmitter Signs (TC-44, TC-45)