Well, think about it. People get into car crashes all the time, and that's with roads and traffic lanes and relatively few judgments to make. Stop, go, changing speeds, changing lanes. Not to mention, if they have to pull over for some reason, they're safely on the ground.

Add another dimension, and all of a sudden they have more decisions to make, and the entire sky to monitor for other vehicles, instead of just the road. And if something goes wrong, they're falling out of the sky.

You could set up aerial traffic lanes and mimic the rules of the road, but someone's always trying to drive in the breakdown lane. They'd do it in flying cars, too, unless you gave the traffic AIs complete control over the vehicles.