Above the I-90 tunnel is a well established residential neighborhood built on glacial fill. In order to stabilize the hillside and keep the tunnel from undermining house foundations, a special technique had to be used to bore the tunnel. The technique called for a ring of small tunnels to be bored through the hillside. The ring was like a round necklace made out of small tunnels around the perimeter of the space that the main tunnel would soon inhabit. Before the main tunnel could be scooped out, the small tunnels were filled with concrete thus creating a solid archway to hold up the hill while the large tunnel was excavated.
This process met that the tunnel had to be round, but the freeway that goes through the tunnel is much wider than it is tall. Fitting the wide freeway into the round tunnel met that there was a lot of room at the top, and the bottom, of the tunnel for things like bike paths. This situation created the opportunity for the bike path at the top of the tunnel while the bottom of the tunnel is filled with all kinds of utility lines, air ducts and service corridors.
After the tunnel, the bike path continues along side I-90 across the Lake Washington Floating Bridge. Yes it is a bridge that actually floats right on the water.
I couldn't find my picture of the floating bridge, otherwise it would be here.
On top of the Mercer Lid, planners have taken advantage of the space, and the exhaust stacks, to make an unusual and interesting park. The bike path, along I-90 continues to wind its peaceful way through these parks with interesting sights.