I'm starting to construct a subway system. My attempt for highways was a big failure, probally due to the fact that I can't build highways through skyscraper zone, else I end up paying hundreds of thousands of dollars.
Got commercial skyscrapers
All it took was a quarter of a million people squished into a 4k by 4k zone
. I'm going for even more, by promoting low-wealth citizens. I hope to have more city density than Tokyo. I don't suppose there's a very high density zone?