From The Urban Blog: Seattle: Why Aren't We New York Yet, pt 1
We've been banging our spoon on the high chair for years about the obvious and inexpensive notion of floating in a 520 replacement span, so it's nice to hear others pipe in (emphasis ours):
"The Washington DOT won't even entertain the notion of building an exact capacity replica of the existing roadway without adding an additional 25% to its footprint. Even though the existing 100% of roadway works just fine I'd say 90% of the time.
Similarly with the SR-520 bridge. This is a floating bridge, quite a technical marvel, quite the thing for Washington State to crow about. SR-520 is the primary corridor which connects us tech-savvy urban dwellers of Seattle, with our great Eastside (suburban) jobs at Microsoft and others. There are times of the year when navigating this roadway is a maddening exercise; and we know the existing structure is reaching the end of its maintainable existence. Therefore we are "planning" for the future.
What do these plans consist of? Well, adding at least one lane in each direction of traffic capacity, for one. Another: Adding a new interchange which would increase capacity even further. And I'm talking about traversing both historical city neighborhoods, and explicitly set aside nature lands (our fair city's arboretum, for one)
We all know the key liability in this whole equation is our floating bridge: yet not once, have I ever heard seriously considered, the notion that instead of "fixing up" the entire eventually to be obsolete corridor, that we'd just replace the existing liability (the bridge) with an identical structure.
Cost-wise, we're looking at around 2 billion dollars to upgrade the 520 corridor. And probably 5 years loss of that highway's capacity. Yet I have never heard talk of building an identical bridge albeit one with no risk of sinking to the bottom of Lake Washington, and floating it out to the existing bridge, and unhooking the existing bridge, and hooking up the new one. "Not an improvement!' you say? I say you're wrong! This bridge probably won't sink for at least 50 years. I'll take those odds.
So the viaduct tunnel is to cost 6 billion dollars, and the 520 replacement, at least 2. Meanwhile, they could spend 2 billion or less putting up a more or less identical viaduct (footprint) and a couple hundred million TOPS replacing the aging bridge. We in Seattle would have money for a better transit system"