WashFlyer also used to have a Union Station route, as well as a DCA service. If it were a question of traffic, I would think jetBlue should have done it. I know lots of people in DC who swear by B6, and maybe so would I if the flights I wanted weren't regularly $100 more than the equivalent United fare.

Anyway, at $16, the downtown coach was only a little cheaper than van service and ran only every half hour. The 5A is $2.50, which you pay for in frequency (~hourly), destinations (not quite downtown), and experience. I wish Metro would add or increase service to Dulles. The bus is often very crowded with airport workers not to mention all the travelers with luggage, and 45 minutes is a long time to be standing.