I've been to Mexico City two times. Both times the traffic has been insane. Remarkably, all the locals I spoke to always attributed bad traffic to "rush hour" — but personal experience indicates that rush hour is all day long.
Everyone I spoke to agreed traffic is a problem, but they all felt it was only a problem at certain times of the day.
Bottom line is 20+ million people in one city (largest in the world) is going to create massive transit challenges.