Here's your animated, soaring, birds-eye-view of Seattle with smooth-as-lube traffic:

Besides the oceans of cement on either end of the tunnel, here's what jumps out to me: It's truly and verily a downtown bypass. And since it would have only four lanes, the same as an average downtown street, it doesn't even let many vehicles bypass downtown. For the cars that exit, the first portal is far, far south of downtown in SoDo (where you'd have to get off the highway, navigate several streets, and make lots of turns to reach the heart of the city) and the daylight returns far north of downtown (where turning off Aurora Avenue N, backtracking, and then crossing the clusterfuck on Denny Way would take forever). Without exits, it just isn't a particularly useful chunk of roadway.

But, man, look at those views!