I hear that a lot about the DC height restrictions, but I wonder if it might not have the opposite effect. Plenty of European cities have tons of density in a low-rise skyline, such as Paris. I wonder if height restrictions actually encourage the 10-story pedestrian-friendly, urban core to keep expanding into the single family home residential areas.

As soon as you cross the Potomac into Virginia there may be high-rises, but everything is built for cars, not transit.

Also, quite a bit of the metro system is cut and cover and quite shallow. A few spots were tunneled deeper to cross waterways.