After seeing the Washington D.C. region leads the country in traffic, one reader of the Washington Post suggests McMansions have contributed to the problem:

Regarding the Feb. 5 news article “Washington again rated worst for traffic congestion in annual study”:

I don’t understand. The entire metropolitan region builds, builds and builds, squeezing  condos onto every block and ruining old neighborhoods with ghastly McMansion and townhouse developments.

Do officials consider quality of life? Don’t they realize how these new homes have a tremendous effect on our local traffic? We have overbuilt this area to death.

It would be interesting to see a study on this. I suspect the real answer is not McMansions over other forms of housing and development but rather the issue of sprawl. McMansions may often be found as part of sprawl but not necessarily; McMansions don’t have to be built on large lots, which leads to more spread out development, and they can be built as teardowns in denser areas. But once sprawl has already happened, it is more difficult to provide effective mass transit (even as the Washington region sees an expansion of Metro service to suburban counties). In other words, McMansions are symptoms of sprawl which leads to a lot of driving and traffic.