The lobbyists argue that the Western Transportation Corridor (WTP) is needed for increased freight traffic at Dulles airport. A study by VA shows that 90 percent of freight is trucked along highways, showing it as a weak argument. With a WTP freight won't increase, however the developers would get another corridor for building more home and subsequently increase East-West traffic.
Also the proposed WTP would drop travelers West of the airport, showing again that it would just be another residential development corridor and not contribute to economic development.
Read on about Freight Traffic (see pages 22-25): http://www.vtrans.org/resources/Northern_Virginia_Connector_I-66.pdf