The state of Michigan uses the 'drive with due regard to the safety of others' rule. My employer uses the 15 mph above posted speed rule. Of course, that's only when it's safe to do so. On some of the back roads you're doing really well if you can safely reach the posted speed...
I'll add to what I said before.
If we are running code in a construction zone or school zone we are required by company policy (not sure about the law) to drive the posted speed limit.
Makes for slow going sometimes considering our two interstates are both undergoing heavy construction to widen them.
I will never ever speed through a school zone, no person's life is worth risking the lives of children.
My partner and I take it a step further and have an unspoken agreement that we go the speedlimit in residential areas as well.
We both also agree on a 85 mph cap on the freeways. Our units will do 95 but I do not want anything to do with blowing a tire and wadding an ambulance up at nearly 100 mph.