In the Bay Area they have something called “Spare the Air Days“. Basically, on days when the air quality is poor (when the “Air Quality Index” goes above 100) most rides on transit in the Bay Area are free. These days are meant to encourage transit and have a pretty strong lingering effect; when I used to take Caltrain, the train was more full all summer starting on the first Spare the Air Day and ending sometime around the end of summer (October or November in San Francisco). Actually, Caltrain seemed to be constantly increasing its ridership as gasoline became more expensive.
Anyway, I think this idea would work in Seattle, make transit free on a few weekdays in the summer, and you’ll find people will want to take transit all day long. I wonder how much it would cost Sound Transit and Metro to implement this kind of system. I’ll make sure to ask in my next meeting.