Union Station to IAD
As per the title, I have to get from Union station to IAD on a Sunday afternoon. What's my best option? I was thinking about a taxi but I know that the DC cabs don't take credit cards.... I was also thinking about getting a car service (towncar), would they pick me up at the entrance to the station? Any help is appreciated! Also how long should it take eg. what are the general traffic patterns? Thanks!