Most people driving across Lake Washington on I-90 have no idea the road is floating. It is. The I-90 floating bridge sits on hollow concrete pontoons, basically giant sealed concrete boxes, and they float the same way a boat does: they push aside enough water to hold up their own weight plus everything on top. Steel cables run from the pontoons down to anchors on the lake bottom so the bridge stays where it's supposed to.
I cross it on my Inside Seattle Engineering tour. Here's the short version.
Why not just build a normal bridge?
Lake Washington is deep, more than 200 feet in places, and under the water there's a thick layer of soft mud before you get to anything solid. Piers would have to go down a long way to hold anything up. Floating the road on concrete pontoons was the practical answer, and it's been working here since 1940.
What keeps it from drifting away
The newer of the two I-90 bridges, the Homer M. Hadley Bridge, has 18 hollow pontoons and 52 anchor cables tying it to the bottom of the lake. Those cables are what keep wind and waves from pushing it around.
The lake also rises and falls about two feet over the year. The bridge rides up and down with it, and the joints where the floating part meets the fixed spans near shore are built to handle that movement.
Two floating bridges side by side
I-90 actually crosses on two floating bridges. The Lacey V. Murrow Bridge opened in 1940 and carries the eastbound lanes. The Hadley Bridge opened in 1989 and carries westbound traffic and now the trains.
The Murrow bridge sank during a storm over Thanksgiving weekend in 1990, while it was being renovated, and was rebuilt by 1993. At 6,620 feet it's the second-longest floating bridge in the world. The longest is the SR 520 bridge, just to the north, which is why I call I-90 one of the longest, not the longest.
There's a train on it now
Since March 28, 2026, Link light rail's 2 Line runs across the Hadley Bridge. It's the first light rail line on a floating bridge anywhere.
Getting trains onto a bridge that moves took some work. Crews ran high-tension steel cables through the pontoons and tightened them, so they act like one stiff barge instead of bobbing separately when a train goes over. They also took out some of the gravel ballast inside the pontoons to add buoyancy. Where the floating spans meet the shore, special track platforms let the rails flex as the bridge moves up and down and side to side.
You can ride it yourself. Take an Eastside-bound 2 Line train from International District/Chinatown Station, and a few minutes past Judkins Park you're out on the lake on the way to Mercer Island. Worth doing once. Check Sound Transit for current fares and times.
Quick answers
Is the I-90 bridge the longest floating bridge in the world? No. SR 520 is the longest. The Murrow Bridge on I-90 is second.
Does the bridge move? A little. It rides up and down with the lake, and wind and waves move it slightly. The anchor cables and the joints at each end handle that.
Can I see it on a tour? Yes. We cross Lake Washington on I-90 on the Engineering tour, along with the Ballard Locks, the SR-99 tunnel and the Ballard Bridge.
See it with me
Crossing the I-90 floating bridge is part of my private Inside Seattle Engineering tour: floating bridges, the Locks, the tunnel, Fishermen's Terminal and the rest of the stuff that keeps this city moving. Just your group, with pickup from downtown and airport hotels, Pier 66, Pier 91, Pike Place Market or Sea-Tac Airport.
More on how Seattle works: Why Seattle's drawbridges open · Why Seattle might be the most engineered city in America · Getting around Seattle on transit
