Population growth and city growth

Hello alltogether. I read a scientific article, unfortunately I dont have it handy, but it stated, that the population of the US, due to the birthrate and the immigration of some 1 million people /year will grow up to 350 mio in 2030, and maybe even over 400 million in 2050. Now my question is, where should all these people live? I mean, now you have 300 million people. You would have to create 20 cities with 5 million people each to accomodate this. 20 cities with about the size of chicago!!!!!!!! What does that meand for the future US? How will it affect the way, the US is organized concerning housing, transportation, public or individual, and workin? The population density wil reach european or even asian numbers in some areas. Any comments>