Parking is $12 bucks in the lots north of the stadium, if I recall. They have shuttle to bring you in or you can hoof it.
Go early and spend a little little time in LODO before you head in. Lots of good restaurants, bars and brewpubs down there. Not really much of a way to get away from post game traffic unless it is a blowout or you leave early.
Point being, if they take rail or a shuttle from the park to a point north, they then can proceed on north without having to retrace their steps and drive through downtown again like they would if they parked south on Sunday when they will be driving north to get home afterwards.
