Working in the industry on these technologies, this is a horrible idea. I've driven vehicles that already have it, and it's nice when it's optional, but would legitimately be a hazard if it was on all the time.
What happens when it's dark and/or rainy, and it reads the 45MPH sign on the side road you were on, but misses the 70MPH sign when you're actually on the highway? It limits your ability to actually accelerate to the flow of traffic as well, since it generally won't change the speed until after you pass it. Or even better, you're doing 70 and it catches a 35MPH on a side road adjacent to the highway? What happens if you just cover the camera and it can't read anything? Does the car just go into limp mode and limit you to 25MPH?
This isn't a hypothetical, I see it happen very, very regularly in even the best systems available. They also probably won't work for the lighted school zone speed limit signs by me, or the express lane type signs.
Map based also eliminates school and construction zones, which is where you want this most,