HOV-2 = High Occupancy Vehicle (2 or more persons per vehicle)

HOV-2 is enforced every weekday morning going into DC on I66. Once it expires at 9am, I66 is swamped with single-occupancy vehicles and the bridge into DC gets backed up a few miles. Since you can't possibly make it onto I66 before 9am, you'll most likely encounter moderate congestion where the airport expressway meets I66 and again at the TR bridge. The congestion usually abates at 9:30-ish or so depending on your luck.

Tell your cabbie to take the fastest route. He might take you through I66 or GW Parkway (hit or miss). Either way, it'll take you about 30+ minutes to make it into DC from IAD which should put you over 9:30 (hopefully) and you can ride traffic-free up Rock Creek Parkway. RCP is uni-directional at rush hour (all 4 lanes headed south/downtown) and becomes two-way again at 9:30 which allows you to jump onto it and make your way uptown and be at the Mass Ave exit (stone's throw from 30th ST) in no time at all. If you make it into the city before 9:30, trying to get to Mass/30 is pretty torturous but your cabbie should know the fastest route (he'll most likely wait till 9:30; fighting Key Br traffic or the E ST expressway exit onto West End is no fun).

Good luck!