I wish there was a setting like this.
I use AP on a road where the limit starts at 50 MPH. I set it to 55 MPH. The limit drops to 35 MPH, and the car drops to 40. The limit goes up to 40 and the car goes up to 45. The limit finally goes back to 50, and the car continues on at 55. It all works great if I get on the road and turn on AP when the limit is at 50. But if I initially turn on AP when the limit is 35 MPH, the car does not raise it's speed when the limit does up to 40 - I have to do that manually.