ColumbiaPete is 100% right on...there was a walkway between the old B and C, since Southwest had flights from both. I remember using that several times when security was backed up one way or the other.

Now, A&B are connected, and C and D are each standalone with no way in from anywhere else.

There are things I dislike about BWI, but long security lines aren't one of them. I don't think I have waited more than 15 minutes for security there in the last 5 years at ANY of the checkpoints. And that included a lot of prime-time (Fri PM) departures.