SamS,
I have been on that road a many of a time and I would highly recommend not doing it. Not only is it one boring strech of highway, you will come darn close to Blacksburg and VA Tech (TRADERS!!!!) and you will never be the same!!! Seriously, coming from Harrisburg on I81, I would cut off a detour in Winchester, VA , taking I 66 to 17 south and going from there. This road is mostly four lane through some great VA towns and will let you out on I95 in Fredricksburg. This would save you a considerable amount of time, get you around the DC area, and still be an enjoyable ride.
If you have any other questions, send me a PM!
Let's GOOOOOO Mountaineers!!!!