Flying cars. The idea has intuitive appeal

just drive like normal, but most congestion problems go away!

https://en.wikipedia.org/wiki/Flying_car

We've made them, but the tradeoffs that you have to make to get a good road vehicle that is also a good aircraft are very large. The benefits of having a dual-mode vehicle are comparatively limited. I think that absent some kind of dramatic technological revolution, like, I don't know, making the things out of nanites, we'll just always be better off with dedicated vehicles of the first sort or the second.

Maybe we could have call-on-demand aircraft that could air-ferry ground vehicles, but I think that with something on the order of current technology, that's probably as close as we'll get.