snuffy: tell me what is the definitive legal situation
This is a discussion forum. You wont get a definitive legal answer on a question like yours here. All you might get are opinions, suggestions and, if you are lucky, someone who thinks they know the answer.
My opinion is that you stick to the speed limits. If you can't do that (I wish I could!), you should turn on your gadget unless anyone in authority asks you to do otherwise.