When I go to big cities, I see an abundance of cars just blocking the streets creating traffic jams and such. I think the reason why is because a ton of people live in the cities to the point where they not only expand "horizontally" like building more buildings in more areas around the city, they also expand "vertically" like building higher and taller buildings like skyscrapers for more people to live in. So what I would like to know is, since so far roads only expand "horizontally", why can't we build roads "vertically" like stacking roads on top of roads like skyscrapers stacking story on top of story?