Since I'm the webmaster at the company I work for, and my e-mail's about the only contact info on there short of writing us a letter, I have to handle a lot of feedback. One of the questions I often get asked is why we don't pursue a roadable version. (Yes, "roadable" is a word. Merriam Webster may not know what it means, but Wikipedia does, and Google certainly returns enough hits for it.) Indeed, it is possible to make an autogyro that you can drive on the road - consider the Pitcarin AC-35 of the past, Larry Neal's Super Sky Cycle that's currently available, or the PAL-V, that's the latest buzz, but hasn't gotten off the ground yet.
While a flying car has long been a dream that we've all wanted, there really are some practical reasons that make it very difficult. For one thing, you have to get power to the wheels, and ideally, disengage the propeller to make ground operations safer. While it's not a huge technical challenge to do this - it's just a gearbox, driveshafts, and clutches - it certainly does add weight.
For another, cars need to have much more collision protection. Aircraft designers don't worry about this because there's not much to hit in the air, but automobiles are constantly in danger of being hit by or running into something. They need the bumpers and other protection that they have. Plus, since they're more apt to smaller "collisions," like shopping carts in the parking lot, they need thicker skins to handle it (plus, a small dent to the leading edge of a rotor is a big deal - it would have to be fixed before you could go fly again, as opposed to a ding in your door, where you cuss that it happened, but just hop in and drive off). Once again, all this adds weight to the vehicle.
Thirdly, since air really isn't very dense, you need something pretty big to act on it to get an aircraft into the air - rotors or wings. You don't want something that big on an automobile sticking out into the other lanes of traffic, so you need some way to stow it, and preferrably a way that keeps it attached to the vehicle, since history has shown that people don't want to have a roadable car where they leave the tail and wings at the airport and drive a detachable car away from there (such as the Taylor Aerocar). To me, a folding rotor seems like the best option to accomplish this, but it's still a good bit heavier than a regular, non-folding rotor. Plus, you'd like to have something that stows pretty quickly and easily - have you ever noticed how many Jeep CJ owners only fold their tops up or down every month or so, even though it only takes about 10 minutes to do?
I hope you're seeing a trend here - weight. Weight is much more of a premium in an aircraft than in a car - you rarely see automobile drivers adding up the weight of passengers, fuel, and any cargo before each trip to make sure they're not over gross weight. Plus, have you ever noticed how chintzy everything seems on an airliner - it's not because the airlines are cheap; it's to keep everything light. For all the issues I discussed above, you can try your best to engineer everything as light as possible, but you can't go too far or you'll be left with a fragile vehicle. Yes, it's possible to make a roadable aircraft, but the compromises necessary add a fair amount of weight, reducing the useful load you can carry. For some people, this reduced useful load would be offset by the greater mobility, but for many applications, it's not worth it.
So, it's this weight issue that I think is the primary reason flying cars never caught on. There are a few other, smaller issues, that I plan to discuss in an upcoming blog entry, "When Will There Be an Aircraft in Every Garage?"
