When must you switch from high beams to low beams?
Within 150 metres of the oncoming vehicle
Switch to low beams within 150 metres of an oncoming vehicle so you do not blind the other driver. Also use low beams when you are less than 60 metres behind another vehicle, unless you are passing it.
Why the other answers are wrong
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Within 30 metres of the oncoming vehicle
Far too close. By 30 metres the glare has already reached their eyes. Keep two numbers straight: 150 metres for oncoming traffic, 60 metres when you are following behind someone.
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Only if the other driver dims theirs first
Your duty does not depend on what they do. Switch to low beams within 150 metres even if their high beams stay on. It is safer when at least one of you can see the road clearly.
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Only after the vehicles have passed each other
That is backwards. The point is to protect the other driver before you meet, so you dim on the approach. High beams can go back on once they are past.
Quick way to remember
There is a set distance from oncoming traffic. It is more than a car length.
Based on the official Ontario MTO Driver's Handbook.
Reading it once is not enough
Drill this question in a scored round and find out what else you are shaky on.
Practise Rules of the Road: The Basics