Computers that are called cars should be on roads, in general. Sometimes they should be in driveways or in garages, but the issue is mostly keeping them on roads while they're in use. The challenge in user interface design that I see is that users of desktops are generally in safe situations where they can pay attention to the computer and ignore their surroundings most of the time, while users of computers in many of these new situations have to pay attention to something outside of the computer's interface (e.g., the road), and can only spare any attention for the computer for moments and only when they can tell that it is safe (clear view of the area and the driver has just looked around). When the first Priuses came out, everybody who tried driving them was driving relatively unsafely simply because they were looking at all of the fascinating information the car provided on the dashboard about its efficiency, and not paying attention to driving. And there are all of the attention problems associated with smart phone use in cars; these can't be ignored in the case where the computer is the car and you're using it any time you drive.