Beauty of the bicycle (+ dog trailer):
1)
no worries about gasoline and gridlock caused when cars run out of gas, as many will before they reach even the Beltway.
2) ability to vacate the city via the
National Mall,
C&O Canal towpath (184 miles going northwest),
Mt. Vernon trail (18 miles headed south),
W&OD Trail (45 miles going west) and the many other bike paths we have in the DC metro area.
http://washcycle.typepad.com/bikemap/DC's "Bikeshare" bikes surely would be part of the exodus.
I'm blessed to have so many hobbies which mesh with evacuation (such as camping, hiking, biking and bikejoring). I believe most of the credible threats against DC (including low-yield nuke or dirty bomb) would make sheltering-in, and least short-term, a better and more viable strategy than immediate evacuation. There will be no immediate evacuation unless the alert goes out at 3:00 a.m. and you happen to be one of the few who know to leave.
..