Miles driven is a less valuable number if the miles are largely along "easy" paths. What interests me is challenging events, perhaps what you call "interesting miles" because the more of those that cars successfully navigate, the more real-world ready the autonomous vehicles will be. There are hundreds of different types of these situations that are common enough that these vehicles will have to react appropriately (construction, detours, officer using hand signals, missing or misleading road paint, inaccurate maps, etc.) I don't know where Google's cars have driven, so they may have encountered these things, but I don't know.