The problem is that any flying object creates a massive invisible wake behind it which means that they can't be packed too closely. Birds have evolved special f

The problem is that any flying object creates a massive invisible wake behind it which means that they can't be packed too closely. Birds have evolved special flight patterns to form a flock so that they don't get knocked out by each others' downdrafts. Air-propelled flying cars can never travel as close together as grounded cars, and consequently don't fix traffic much at all. If you were to invent some sort of antigravity device, this might be different, but I'm not going to hold my breath for that!