Just to add to what Charles said above, it's my impression that radar cruise works more on distance than speed, or perhaps they're equally important.
Say you're going through a stretch of roadworks with a 50mph limit on the motorway. You enter the roadworks, set the speed at 50mph and away you go. If the vehicle in front of you is only doing 40mph you'll soon catch it up, but radar cruise will slow your car down to maintain a certain distance between you and the one in front. If the car in front speeds up, so will yours. Or if you pull out from behind the one doing 40, your car will then speed up to whatever you've set, ie, 50 in our example.