The idea of cruising 15km/h over the limit is absolutely crazy to me. That will get you 3 points and a minimum $500 fine here. We have "average speed zones" too!

Where I live travelling at that speed will get you passed by every cop and state trooper driving on the same road. A lot comes down to local norms and enforcement.

In Alabama on the interstates and highways the rule of thumb is: "8 you're great, 9 you're mine."