Waiting on a freeway entrance ramp in rush hour, you find a traffic signal partway down the ramp — one head high on the right and a second mounted unusually low on the left. Three cars are ahead of you and it goes green. How many of you get through?

Correct answer: D. Just you when it is your turn — a ramp metering signal releases one vehicle per lane per green

One car per lane, per green. These are ramp metering signals, and Book 12 explains they exist "to meter or gate the volumes of traffic, not to allow right-of-way to other vehicles at an intersection" — the metering "is accomplished by allowing only one vehicle per lane per green indication through the location." So the queue moves one car at a time, spacing vehicles out as they join the freeway instead of letting a platoon merge at once. The odd low head is required by HTA Regulation 626 subsection 1(6), which puts one signal on the left "not less than one meter above the level of the roadway" and one on the right at not less than 2.75 metres; the low one is there because the stop line sits so close to the signal that a normally mounted head would be above your windscreen by the time you stopped.

Source: MTO Driver's Handbook: Ontario Traffic Manual, Book 12 — Traffic Signals (July 2024), Section 2.7 — HTA Regulation 626, Subsection 1(6), Ramp Metering Signals