The future of vehicles will be modular. Your passenger compartment will be a module, your drive-train, your energy source (probably a battery pack). This will solve all kinds of problems with weight and utility in vehicle usage.
This is what will make flying cars possible. Your passenger compartment and engine can mate to a ground transport package for driving around town. Or it can mate to turbofans for flight. Have “miniports” for take-off/landing stocked with car-mode transport packages.
You drive from your home to a miniport, where you switch from the ground transport package (wheels and transmission) to an air transport package (turbofans and appropriate gearing).
You fly between miniports much like you commute from one train station to another - say, distances of 20 miles to 30 miles. On landing, you leave the air transport package and mate to a ground transport package to drive to your final destination.
Solves issues of weight for flying, and packaging the lift mechanism when driving like a car.
The package swap at a miniport could include a battery swap to give you a full charge. If battery range and weight reduction get you to a range of, say, 100 miles, for longer trips you could land every 80 miles or so to do a battery swap. A five minute delay once an hour to have unlimited range for your flying electric car.
This could also effectively eliminate the need for freeways. If you’re never more than 15 miles or so from your destination in a metro area, you don’t really need a freeway. For journeys between metro areas, miniport/battery swap stations at 80% of the full range would let you hop your way to your destination.
Think about the amount of real estate this would free up in metropolitan areas. Very expensive real estate. That alone could finance one heck of a lot beyond the infrastructure needed.
And if you really need to go some serious distance? Your passenger compartment can be mated to a train, or an commercial jet’s fuselage. Travel in your car on high speed rail, or on a cross-country flight.