The DC Metro has a daily ridership of about 700,000 trips. This is nothing to sneeze at, and it is impressive. However, I'm not positive how they calculate a trip, but my guess is that it's a card swipe entry and exit. Most people are traveling to and from work, so that is likely half that number in unique people. So say 350,000 - 400,000 a day. On the other hand, I-270 through Maryland sees about 200,000 cars daily. That's one highway - with at least some cars carrying more than 1 person. Interstates 66 and 95, US route 50, MD route 295, and VA route 267 all carry similar amounts of traffic. And lets not forget the other minor routes and streets that carry additional traffic. Your assertion that it "would be intractable for ANY highway system" to handle this is absurd - it already does.

While I too find some scorn for the McMansion culture, it is not really a local developers job or role to plan for medium to long distance infrastructure. I don't think it would even be legally possible for a private entity to take on a highway project that crossed multiple counties and/or states without a massive amount of support from local and state governments.

Also, I wouldn't call suburban and rural development an anti-pattern. Unless you want the cities to be double the density they already are, some people are going to have to live in the suburbs and rural parts of the country.