My car does this aswell, and quite well actually, especially combined with ACC.
There are however instances where it gets confused, especially when there are 2 visible speed limits (like 120 for cars and 70 for trucks), it sometimes automatically takes the lowest one out of precaution.
It doesn't slam on the brakes though, but it does slow down noticably, which usually gives me enough time to override it and set it back to what it was.
The audible beeps are a bit annoying though, especially if you dare stray 1 mm over the line it gives all sorts of beeps, visual warnings and a vibration in the steering wheel.... A bit much when you're driving in some roadworks where the road is almost as narrow as the car.
Despite having the car for 2 years I've never really tried to find the off setting, I kinda learned to live with it. I'll try to turn everything off and see if it remembers...
Also you CAN give a % or an absolute value in km/h as extra leeway before it starts whinging, but since my car's speedo is pretty accurate (3 or 4 kph off at 120) I kinda like the extra buffer ...