Mass transit is supposed to reduce traffic congestion by getting more people out of their cars. That's the theory, anyway. But a detailed engineering analysis of traffic in Reston by 2030 around the three new Silver Line Metrorail stations by the Fairfax County Department of Transportation indicates otherwise. The county's baseline planning scenario -- "Scenario E" -- shows gridlock at two key intersections: Sunrise Valley Drive and Reston Parkway, and Wiehle Avenue and Sunset Hills Road.

That's because the scenario envisions doubled density around the Metro stations, shoehorning 50,000 new residents and 100,000 more workers into an area that currently contains 6,000 residents and 70,000 workers. As the DOT analysis suggests, the newcomers won't all be walking to the subway. In fact, "peak of the peak" traffic at these two intersections will ground to a standstill for nearly four minutes, creating backups in the surrounding neighborhoods and beyond.

That's even if the county adds all the new turn lanes and other upgrades that it has planned for the intersections but hasn't yet figured out how to fund. The analysis doesn't take into account the tens of thousands of additional vehicles expected to swarm local roads to avoid punishing tolls on the Dulles Toll Road.

The horrific prospect of having to pay for a very expensive transit system and accompanying infrastructure only to find it will dramatically increase traffic congestion prompted Colin Mills, president of the Reston Citizens Association -- a longtime supporter of Dulles Rail -- to urge Patricia Nicoson, chairwoman of the Reston Master Plan Special Study Task Force, to consider "Scenario G" -- which would reduce permitted density around the stations and adjust the commercial-residential ratio to more "realistic levels."

Unfortunately for Restonites, the Silver Line is unlikely to deliver the "right densities of the right mix in the right places" that its political advocates promised. In September, the Fairfax Board of Supervisors rejected citizen complaints and its own staff recommendation and approved a new 23-story mixed-use building on Reston Parkway. One resident quipped that the new high-rise will be so far from the Metro station that only a "cheetah or a marathon runner" could walk there in 15 minutes. Most of the new tenants will likely drive, adding to the coming gridlock.

Contrary to what Reston residents have been led to believe, brutal congestion is in their future, and the Silver Line will do nothing to avert it.