I was trying to find out what is going on outside Marshall at Crest Hill (Rt 647), but still don't know.  
However, in searching, I see that VDOT is planning a roundabout heading south of Marshall on Rt 17/I-66.  
At Carters Run there will be an R cut with a U turn rendering Carters Run essentially unusable as a northbound cycling road.  
Carters Run southbound would still be accessible after navigating a potentially tractor-trailer filled roundabout. I cycled Carters Run, which I love for its rollers, low traffic, and views, the other day and am sad to see this on VDOT's site.  
I tried to attach photos of planned interchange from VDOT's site, but I got a nastygram back saying the attachments are too large. ;-) 
Tailwinds,   
mv