Rodney Brooks on why your promised flying cars (or hyperloops or autonomous vehicles) aren’t likely to happen any time soon:
The difference between the possible and the practical can only be discovered by trying things out. Therefore, even though the physics suggests that a thing will work, if it has not even been demonstrated in the lab you can consider that thing to be a long way off. If it has been demonstrated in prototypes only, then it is still distant. If versions have been deployed at scale, and most of the necessary refinements are of an evolutionary character, then perhaps it may become available fairly soon. Even then, if no one wants to use the thing, it will languish in the warehouse, no matter how much enthusiasm there is among the technologists who developed it.
The Rodney Brooks Rules for Predicting a Technology’s Commercial Success