But Steve, how could one possibly turn the engine off and then on again while whizzing along on the motorway for example?

My husband was desperate for one of those Sat-Nav things. (In spite of the fact we already know every inch of Norfolk and never go anywhere else in our car!) One Crimbo I got him one and he was thrilled. We went straight out towards North Norfolk, with the ruddy thing burbling away. (we'd typed in the postcode of the holiday barn we used to clean)
We decided to do exactly as the lady said, in order to test it. She obviously hadn't the least idea and we ended up heading for a remote farmhouse, driving up a track to the farmer's house. She always seemed to have second thoughts, saying, "When it is safe to do so, make a U turn."
It now sits in a drawer.
One thing that amused me was that one could change the accent. We rather liked the Irish lady...