What is something that's crazier than the wildest dreams of any ~1970s nerd, but exists today? I'll start with an easy one:

Well, still no flying cars.

We can barely handle non flying cars. Can you imagine the amount of mini 9/11s drunk drivers flyers will cause?

The chance of collision in 3d space is so much lower than 2d.

It’s a lot harder to paint lanes in the sky