Do you have to drive below the speed limit in bad weather?
Below the limit: slow enough to stop safely for the conditions
The posted number is a maximum for ideal conditions, not a target. You must drive at a speed that lets you stop safely, which means below the maximum in bad weather, heavy traffic or construction zones.
Why the other answers are wrong
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Exactly 80 km/h, since that is the posted limit
The posted number is a ceiling for good conditions, not a target. In heavy snow you still have to be able to stop safely, and 80 on a slippery road is far too fast for that.
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Up to 10 km/h over, to keep up with traffic
Matching traffic never lets you pass the posted maximum. Going over is illegal in any weather, and in heavy snow it is the opposite of what conditions call for.
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Any speed, as long as you use your hazard lights
Hazard lights warn others that your vehicle is stopped or in trouble. They do not raise or cancel a speed limit, and the safe stopping rule still applies.
Quick way to remember
The posted limit is a ceiling for good conditions, not a goal.
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