[anonporridge]

Mass use of flying cars is fundamentally infeasible on any realistic timescale for a huge list of reasons beyond energy inefficiency. Accidents are catastrophic. The noise would be insane. Minor body damage that wouldn't matter for a car could cause the whole thing to crash. You'd need a complex traffic control system that would never be allowed to miss a beat without risk a Kessler syndrome like event.

And in your water example, building a bridge would be drastically better for a high traffic use case.

In cases where you can't justify infrastructure build out, it will still be more efficient to use all terrain ground vehicles.

And no, hovering in the air and fighting gravity ever moment of active travel will never be anywhere close to the efficiency of a rolling vehicle.