I don't want to encourage speeding but they don't know where you are so nothing less than 70mph should be an issue with a black box. It's more to do with harsh acceleration and cornering I believe.
I'm sure the black boxes are all different but my son's one definitely did know the speed of the actual road (like a sat nav would). One plus of this is that, in the event of your car being stolen, the insurance company can use the black box as a tracker. Also some companies will ring the emergency services if they detect you've been in an accident (read about this on some websites but it's not every company).