Phoenix isn't going to pass Detroit anytime soon. For one, by the time Phoenix reaches 5 million people, Detroit will have absorbed adjacent metros such as Jackson, Lansing, and Toledo creating a CSA of more than 7 million.
BTW, awhile ago I tried creating several "urban regions". The point wasn't to show areas of continuous urban development as that is what the current "urban areas" are for. The point was to show stretches of land where many large urban areas developed in close proximity and have a somewhat cohesive identity. The biggest example is the Northeast Megalopolis. Other examples were the Southern Piedmont, the Florida Peninsula, The Rust Belt, Norcal, Socal, etc.
The only two that I completed were the Northeast Megalopolis and the Southern Piedmont. I can't remember what the population of the Northeast Megalopolis was, but I believe it was over 50 million.
