Generally (barring accidents, which screw everything up), if you leave from BWI by 1 p.m. heading west, traffic won't be horrible (it'll only be bad). Except Fridays, sucks pretty much all day.
Also, you have to be aware of whether or not the Nationals/Redskins/Caps have a home game that day, and when it is, as that can affect traffic greatly from about 2-3 hours before the event. Major concerts too, though those are less regular. And weather events, since the morons around here only know how to drive in nice weather (and even then, a bit iffy), add hard rain or a touch of snow and traffic snarls ensue.