eight hours of congestion a day

Washington, D.C. has, apparently, the third worst traffic in the nation.

I don't know D.C. well, but based on what I've seen on the Beltway, I'm not at all surprised. I'm normally not a skittish driver, but there's something about that road that gets my palms all sweat.

So here's my question. D.C. has a rather good subway system. Does it run at capacity? If not, why don't more people use it?