B the Flying Car Kickstarter project... NEAT gif
Adam Rifkin stashed this in SHUT UP AND TAKE MY MONEY!
Multirotor aircraft rely on changing the speed of electric motors hundreds of times a second to remain level. A computer sends the signals to the motors after interpreting gyroscope and accelerometer data.
When you scale this design up you need massive batteries and also have a lot of added weight in the form of people. Also the momentum of the larger motors makes speed changes difficult. It would probably be possible, but you would need more smaller motors, so a quadcopter would be pretty hard.
You could have the motors run at a constant speed and change the collective pitch of each prop like a helicopter does. This would just be linkage and could probably move quick enough to control the craft. Even then with current battery technology you would have very short flight times.
While it is not a car, here is a human powered one:
I wonder if he could transport Tacos in his Tacocopter?