Montgomery County Maryland has no limited access freeways that lead into DC. There is the Clara Barton Parkway, a limited access 4-lane parkway at the extreme western edge that ferries traffic to Georgetown with few at-grade intersections, but that's it. Nearly one million people live in Montgomery County, but none of them are hopping on a freeway to get to work Downtown in the morning.
I'm not sure if this is a chicken-and-egg issue, but I'm quite sure that this is related to the fact that the Metro's Red Line, which terminates both ends in Montgomery County, has the highest capacity and ridership of any single line. It shares no track with any other lines. Silver Spring and Bethesda have approximately 70,000 residents in each of their downtowns, and yet not a single freeway runs through them. And in each of those downtowns, there is only one Metro station.
Why then, shouldn't Tyson's Corner be able to thrive without freeway widening once it gets its FOUR Silver Line stations? Fairfax County is planning on scaling back its redevelopment because of potential stress on the freeways 40 years from now.
Their rationale is simple: many people will still drive. Well, sure. Especially if you widen the freeways. And especially if the Silver Line is shoehorned through a huge choke point at Rosslyn, where three lines will be sharing one track.
Perhaps Fairfax County should be considering the enhancement of Metro capacity instead of the worrying about freeways. Montgomery County has an assortment of redeveloped edge cities that don't have any freeways connecting their downtowns with DC. If the Beltway is the worry, why not seek to invest in a transit connection between Fairfax and Montgomery Counties?
People drive primarily because driving has been subsidized into being the best option. If Tyson's Corner is looking to progressively remodel the region, perhaps they should think outside the box and reimagine their transit options.