Here's a post I left in the Fractional section:
Back in my LJ days we had several 4+ hour delays west bound out of Dulles/National/ADW. We were told it was a traffic jam in the sky D2 the increase/upsurge in regional traffic, mainly going to and from Chicago.
Also, in the C-17, I've had several metering issues south and north of DC b/c of Washington ARTCC congestion.
In a lot of cases, the new regionals definitely play a part b/c a small BE, DA, or ERJ's taking as much space in IFR (Potomac Approach) as a 777 (not withstanding wake issues).