As someone who has never volunteered to come up to help... (translate: take this for what it's worth )
It seems that one thing to be considered in selecting a road is the sign and the publicity and the good will behind the task. Will you have more impact in in a Loudoun neighborhood of open carriers, or in Fairfax, where many of the residents and out-of-state visitors who use that road literally think that all gun owners are criminals?
Taken to its logical conclusion, what if you could somehow sign up for the 1600 block of Pennsylvania Avenue in DC? You can't carry there yet, but you could show up and show that you are good citizens. Think of the impact a sign in front of the White House with opencarry.org or VCDL.org would have.
Just a thought.