We have those on the freeway, and last year they raised the minimum speed from 45 to 55. A bit amusing I think. If you're an absolute law-abider, you pretty much have to break it either before or after the sign. It goes from "speed limit 55" to "minimum speed 55", so technically you would have to cross the sign at 55 dead on. Same thing coming back, it goes from "minimum speed 55" to "speed limit 55".
I don't care, if it is winter and the road is bad, I won't even go 55. I certainly wouldn't on any other road in the same weather. The physics of driving on ice don't care what road I am on. They also used to have these signs that said "ice and snow, take it slow", which they also removed when they raised the minimum speed.
Once I was on the freeway and it was bad out, going about 45, and a cop came up behind me. He stayed on my tail for a couple minutes then I watched him do a 180 in the road and go into the ditch, thankfully he didn't get up next to me or in front of me first.