Chuck, Wash DC writes:
I've lived in Washington for over 50 years and I don't think I've ever once heard anyone refer to it as the "Metroplex." Dallas-Fort Worth is often referred to as the "Metroplex," and I believe it is the only place in the country where people use that term.
I have live in DC for only 40 years --- but I hear it all the time.
(And I put a picture in my response.)