I've heard that 10% over is allowed. That's not official of course and you could get a ticket for 51 in a 50 zone but I've never heard of it.
I used to live in Edmonton which is a fairly big city. There seemed to be different rules during rush hour. Everyone was driving the same speed and (shall we say) a bit fast. And I never saw a radar trap during rush hours, or a crash either.
I suppose the theory is that working people are mostly good drivers, and everyone is driving on a familiar route, and in a good vehicle so the police should just stay out of the way and let them get to work.
The bad drivers seemed to come out later, the ones who don't stop at 4 way stops, or change lanes without looking, etc. Between rush hours the speed laws were enforced.