Chris - some of the roads I ride out in the western section of Loudoun County are like that - 40-45 mph limit, no shoulder (thank you NOT! VDOT!). I use a helmet mirror (Hubbub bikes, recommended; gives me much better warning sometimes) and I will run a bright taillight (Planet Bike Superflash or greater) on strobe for a bit more daytime visibility. Only time I've been "buzzed" was when I was riding with a group of 6-8 riders (single file) and the driver must've been having a bad day.

Check the PPTC cue library, ridewithgps, mapmyride, etc. for rides starting in/near Gaithersburg. Also check with local bike shops. And the DC Rad recommendation.