3D Mobility

So how long do you think it will be before we have flying cars? 3D mobility instead of only the 2D. Obviously the first question is when will they start making cars which fly. Moller International is selling a prototype of their flying car, called Moller M400 Skycar. This wondercar has a max speed of 375 mph and a fuel efficiency of 20 miles per gallon. Quite practical no? Anyway due to some finance problems this prototype is being sold on ebay with a starting price of 4m$. Not bad.
Btw, here is a nice history of flying cars.

Now say we have the car, then what? are we going to start ourdrive on the sky highway?
Nope! That would be too easy. So for today, I am trying to think of all the challenges if we were to start using hover and flying cars.

1 - (and possibly the biggest challenge?) Air traffic!
I mean how tightly controlled are airport traffic regulations and imagine transporting this onto everyday roads. Air traffic regulations must incorporate:
1.1 Do we have layers of streets or will streets be vertical lines?
1.2
2 - ease of use and affordability
3 - environmental issues, like noise and pollution
4 - safety of people on the ground. Is the sky falling?
5 - sky cars need to be even more robust than street cars - engine problems in a sky car can be fatal!

Can you think of other challenges?

Meanwhile here is a quote from Calvin and Hobbes:

Hobbes: "A new decade is coming up."
Calvin: "Yeah, big deal! Hmph. Where are the flying cars? Where are the moon colonies? Where are the personal robots and the zero-gravity boots, huh? You call this a new decade?! You call this the future?? HA! Where are the rocket packs? Where are the disintegration rays? Where are the floating cities?"
Hobbes: "Frankly, I'm not sure people have the brains to manage the technology they've got."
Calvin: "I mean, look at this! We still have the weather?! Give me a break!"