near as i can tell, you have to do your own leg work. but wsdot has a traffic map archive, so you can go back to any date/time and see what traffic was like:
http://www.wsdot.wa.gov/Traffic/Seattle/Flowmaps/Archive/
and here is the flow map data for daily number of cars on seattle roadways:
http://www.seattle.gov/transportation/tfdmaps.htm
but my experience is that if you leave home after 6:45 am on weekdays, you’ll be in slow west seattle bridge traffic, and it usually doesn’t let up until 9:30 or 10.
coming home, it obviously depends on which route you take.
if i’m not on 99 or I-5 headed for the west seattle bridge by 3 pm, i’ll use surface streets like first avenue to the bridge, second avenue to the columbia street entrance on 99, or columbian way. but if you’re south of downtown, traffic into west seattle is pretty easy, even during rush hour.
good luck.
