...The graphs show why traffic has been improving: over the years, the number of offices (in the Rosslyn-Ballston corridor) has been decreasing relative to the number of residences so more people are able to live near their work. This improvement was made possible by requiring residential development to occur before or simultaneously with commercial and allowing a larger FAR for residential than for commercial. . . Because for the past 20 years the ratio of the added non-res-to-res has been 0.6, traffic has improved substantially.
The full report is offered below. Caution: It's wonky!