How Important Is Congress to the DC Economy?

I was running late last night, forcing me to catch a cab outside the Watergate around six.  Normally, there's a decent wait for cabs around then, as there's a lot of demand.  But last night, there were four or five drivers who had obviously been parked there for a while, because they'd gotten out of their cars and were smoking and chatting.

"Slow night?" I asked the driver.

"Very slow." He said.

"is it slow here at the Watergate, or slow everywhere?"

"Slow everywhere!" he said.  "Congress is out of session."

Which is, if you think about it, rather extraordinary.  The daytime population of the District is about a million people, of which about 400,000 are commuters coming in from the suburbs.  The sudden disappearance of 535 congressmen shouldn't have such an enormous effect on the number of taxis taken, even if they take all their staffers with them.  

Yet it isn't just the taxis; restaurants experience slowdowns, and my commuter friends tell me that traffic is noticeably better when Congress is out of session.  This suggests what an enormous volume of secondary and tertiary transactions those 535 people must be driving.  When people aren't flying in from town to meet with their congressmen, taxis are plentiful and it's easy to score a seat at the hot new restaurant.  When they're back, we DC residents have to eat at home and take the bus.  There's a metaphor in there, somewhere--or at the very least, an interesting little insight into the local economy.