IAD has single security for all the terminals. (Well, actually a couple of security checkpoints, but they all lead to everywhere).


DCA currently has separate security for the different terminals and, in fact, for multiple piers of Terminals B/C. There is a post-security shuttle bus between the two piers of Terminal C, but I don't know of one between the two piers of Terminal B. And nothing connects to Terminal A.

There are plans to change that in the future so DCA will have unified security, but I am not sure of the schedule.