Is DC/Balt different from existing two city metros

Is the Washington-Baltimore metro area fundamentally different from any other metro area in the country? Here's my perspective (and I'm not sure if it is right or wrong): First of all, no two major cities are as close to each other as Washington and Baltimore that were once thought of us as separate metro areas. In other words, that 45 miles or so is the closest distance between major US cities that developed as on their own. Thus Baltimore and Washington have a different relationship between each other than such places as: Dallas and Fort Worth San Francisco, Oakland, and San Jose Minneapolis and St. Paul That's because the Metroplex, Bay Area, and Twin Cities are long standing metro areas that have been bi- or tri-modal for a long period of time. In essence, in each of these 3 metros, whether you were in one city or the other, you were in the same place. Not so with Baltimore and Washington. I look at DC/Balt. being the model for future such developments (although nothing comparable exists today). By future, I'm thinking: Chicago-Milwaukee LA-SD New York-Phila. These adjacent metro areas have about twice the distance between each other (about 90 mi) as DC/Balt, but further in-fill will create a similiar situation to what we see in DC/Balt today. Am I on to something......or way, way off base?>