I'd actually argue the contrary - viewing the map in transit is for the most part useless (not to mention dangerous if you're also the driver), and I'd assume most people rely on the audio cues rather than actually using the map. I would also guess that users would perhaps only look at it either prior to departure to get a general idea of what their route is like or when they're very close to their destination to look for something like parking or other landmarks to help navigate once near the location. (e.g., trying to navigate a new city and dealing with one-way streets)
